diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index f00ebc07b9..9c2cb51034 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -96,6 +96,17 @@ Test samples are kept in `/test/xxx/samples` folder.
 1. To run test, run `npm run test`
 1. To run test for a specific feature, you can use the `-g` (aka `--grep`) option. For example, to only run test involving transitions, run `npm run test -- -g transition`.
 
+##### Running solo test
+
+1. To run only one test, rename the test sample folder to end with `.solo`. For example, to run the `test/js/samples/action` only, rename it to `test/js/samples/action.solo`.
+1. To run only one test suite, rename the test suite folder to end with `.solo`. For example, to run the `test/js` test suite only, rename it to `test/js.solo`.
+1. Remember to rename the test folder back. The CI will fail if there's a solo test.
+
+##### Updating `.expected` files
+
+1. Tests suites like `css`, `js`, `server-side-rendering` asserts that the generated output has to match the content in the `.expected` file. For example, in the `js` test suites, the generated js code is compared against the content in `expected.js`.
+1. To update the content of the `.expected` file, run the test with `--update` flag. (`npm run test --update`)
+
 #### Breaking changes
 
 When adding a new breaking change, follow this template in your pull request:
diff --git a/site/content/examples/20-7guis/05-7guis-crud/App.svelte b/site/content/examples/20-7guis/05-7guis-crud/App.svelte
index f55aeb0d83..aec623d2a4 100644
--- a/site/content/examples/20-7guis/05-7guis-crud/App.svelte
+++ b/site/content/examples/20-7guis/05-7guis-crud/App.svelte
@@ -2,18 +2,9 @@
 
 <script>
 	let people = [
-		{
-			first: 'Hans',
-			last: 'Emil'
-		},
-		{
-			first: 'Max',
-			last: 'Mustermann'
-		},
-		{
-			first: 'Roman',
-			last: 'Tisch'
-		}
+		{ first: 'Hans', last: 'Emil' },
+		{ first: 'Max', last: 'Mustermann' },
+		{ first: 'Roman', last: 'Tisch' }
 	];
 
 	let prefix = '';
@@ -39,7 +30,9 @@
 	}
 
 	function update() {
-		people[i] = { first, last };
+		selected.first = first;
+		selected.last = last;
+		people = people;
 	}
 
 	function remove() {
diff --git a/site/package-lock.json b/site/package-lock.json
index 2f8008911f..09bd67cf1e 100644
--- a/site/package-lock.json
+++ b/site/package-lock.json
@@ -1291,17 +1291,23 @@
       }
     },
     "@sveltejs/svelte-repl": {
-      "version": "0.1.11",
-      "resolved": "https://registry.npmjs.org/@sveltejs/svelte-repl/-/svelte-repl-0.1.11.tgz",
-      "integrity": "sha512-z0Cv9fNggmL6C2bphYa2A5Bku91U3ukCmJTh9pFS68X0sJxDFJPsBgxAmWhrOfOe7NdGvm/s+fBEOo8cce66/w==",
+      "version": "0.1.13",
+      "resolved": "https://registry.npmjs.org/@sveltejs/svelte-repl/-/svelte-repl-0.1.13.tgz",
+      "integrity": "sha512-griMkrRRzAQq22awKaBN5cewGly4xeeo8qpDs8ZhQxmEk5TcX3dMhIGMLuPTSv3Yr0s2c6TDJXcN+ORJn//fpQ==",
       "dev": true,
       "requires": {
-        "codemirror": "^5.48.4",
-        "estree-walker": "^0.6.1",
+        "codemirror": "^5.49.2",
+        "estree-walker": "^0.9.0",
         "sourcemap-codec": "^1.4.6",
         "yootils": "0.0.16"
       },
       "dependencies": {
+        "estree-walker": {
+          "version": "0.9.0",
+          "resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.9.0.tgz",
+          "integrity": "sha512-12U47o7XHUX329+x3FzNVjCx3SHEzMF0nkDv7r/HnBzX/xNTKxajBk6gyygaxrAFtLj39219oMfbtxv4KpaOiA==",
+          "dev": true
+        },
         "sourcemap-codec": {
           "version": "1.4.6",
           "resolved": "https://registry.npmjs.org/sourcemap-codec/-/sourcemap-codec-1.4.6.tgz",
@@ -3368,12 +3374,6 @@
       "requires": {
         "@babel/helper-module-imports": "^7.0.0",
         "rollup-pluginutils": "^2.8.1"
-      },
-      "dependencies": {
-        "estree-walker": {
-          "version": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
-          "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w=="
-        }
       }
     },
     "rollup-plugin-commonjs": {
@@ -3411,10 +3411,6 @@
         "rollup-pluginutils": "^2.8.1"
       },
       "dependencies": {
-        "estree-walker": {
-          "version": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
-          "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w=="
-        },
         "resolve": {
           "version": "1.11.1",
           "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.11.1.tgz",
@@ -3458,12 +3454,6 @@
         "rollup-pluginutils": "^2.8.1",
         "serialize-javascript": "^1.7.0",
         "terser": "^4.1.0"
-      },
-      "dependencies": {
-        "estree-walker": {
-          "version": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
-          "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w=="
-        }
       }
     },
     "rollup-pluginutils": {
@@ -3473,14 +3463,6 @@
       "dev": true,
       "requires": {
         "estree-walker": "^0.6.1"
-      },
-      "dependencies": {
-        "estree-walker": {
-          "version": "0.6.1",
-          "resolved": "https://registry.npmjs.org/estree-walker/-/estree-walker-0.6.1.tgz",
-          "integrity": "sha512-SqmZANLWS0mnatqbSfRP5g8OXZC12Fgg1IwNtLsyHDzJizORW4khDfjPqJZsemPWBB2uqykUah5YpQ6epsqC/w==",
-          "dev": true
-        }
       }
     },
     "safe-buffer": {
diff --git a/site/package.json b/site/package.json
index dc32005555..d08aa1f742 100644
--- a/site/package.json
+++ b/site/package.json
@@ -36,7 +36,7 @@
     "@babel/runtime": "^7.6.0",
     "@sindresorhus/slugify": "^0.9.1",
     "@sveltejs/site-kit": "^1.1.4",
-    "@sveltejs/svelte-repl": "^0.1.11",
+    "@sveltejs/svelte-repl": "^0.1.13",
     "degit": "^2.1.4",
     "dotenv": "^8.1.0",
     "esm": "^3.2.25",
diff --git a/src/compiler/preprocess/index.ts b/src/compiler/preprocess/index.ts
index 2faa3b97f2..4549517fbd 100644
--- a/src/compiler/preprocess/index.ts
+++ b/src/compiler/preprocess/index.ts
@@ -94,8 +94,12 @@ export default async function preprocess(
 	for (const fn of script) {
 		source = await replace_async(
 			source,
-			/<script(\s[^]*?)?>([^]*?)<\/script>/gi,
+			/<!--[^]*?-->|<script(\s[^]*?)?>([^]*?)<\/script>/gi,
 			async (match, attributes = '', content) => {
+				if (!attributes && !content) {
+					return match;
+				}
+				attributes = attributes || '';
 				const processed = await fn({
 					content,
 					attributes: parse_attributes(attributes),
@@ -110,8 +114,11 @@ export default async function preprocess(
 	for (const fn of style) {
 		source = await replace_async(
 			source,
-			/<style(\s[^]*?)?>([^]*?)<\/style>/gi,
+			/<!--[^]*?-->|<style(\s[^]*?)?>([^]*?)<\/style>/gi,
 			async (match, attributes = '', content) => {
+				if (!attributes && !content) {
+					return match;
+				}
 				const processed: Processed = await fn({
 					content,
 					attributes: parse_attributes(attributes),
diff --git a/src/runtime/ambient.ts b/src/runtime/ambient.ts
index b094056c59..09cd9b72f2 100644
--- a/src/runtime/ambient.ts
+++ b/src/runtime/ambient.ts
@@ -1,19 +1,3 @@
 declare module '*.svelte' {
-	type Props = Record<string, any>;
-
-	export default class {
-		constructor(options: {
-			target: Element;
-			anchor?: Element;
-			props?: Props;
-			hydrate?: boolean;
-			intro?: boolean;
-		});
-
-		$set(props: Props): void;
-		$on<T = any>(event: string, callback: (event: CustomEvent<T>) => void): () => void;
-		$destroy(): void;
-
-		[accessor: string]: any;
-	}
+	export { SvelteComponentDev as default } from 'svelte/internal';
 }
diff --git a/src/runtime/index.ts b/src/runtime/index.ts
index 0973b93f1e..e6c0c916f2 100644
--- a/src/runtime/index.ts
+++ b/src/runtime/index.ts
@@ -8,5 +8,6 @@ export {
 	setContext,
 	getContext,
 	tick,
-	createEventDispatcher
+	createEventDispatcher,
+	SvelteComponentDev as SvelteComponent
 } from 'svelte/internal';
diff --git a/src/runtime/internal/dev.ts b/src/runtime/internal/dev.ts
index 404d0b643a..cb60131166 100644
--- a/src/runtime/internal/dev.ts
+++ b/src/runtime/internal/dev.ts
@@ -79,8 +79,24 @@ export function set_data_dev(text, data) {
 	text.data = data;
 }
 
+
+type Props = Record<string, any>;
+export interface SvelteComponentDev {
+	$set(props?: Props): void;
+	$on<T = any>(event: string, callback: (event: CustomEvent<T>) => void): () => void;
+	$destroy(): void;
+	[accessor: string]: any;
+}
+
 export class SvelteComponentDev extends SvelteComponent {
-	constructor(options) {
+	constructor(options: {
+		target: Element;
+		anchor?: Element;
+		props?: Props;
+		hydrate?: boolean;
+		intro?: boolean;
+		$$inline?: boolean;
+    }) {
 		if (!options || (!options.target && !options.$$inline)) {
 			throw new Error(`'target' is a required option`);
 		}
@@ -103,4 +119,4 @@ export function loop_guard(timeout) {
 			throw new Error(`Infinite loop detected`);
 		}
 	};
-}
\ No newline at end of file
+}
diff --git a/test/css/index.js b/test/css/index.js
index 1f90d243be..dc17314597 100644
--- a/test/css/index.js
+++ b/test/css/index.js
@@ -1,6 +1,6 @@
 import * as assert from 'assert';
 import * as fs from 'fs';
-import { env, normalizeHtml, svelte } from '../helpers.js';
+import { env, svelte, setupHtmlEqual, shouldUpdateExpected } from '../helpers.js';
 
 function try_require(file) {
 	try {
@@ -37,6 +37,10 @@ function create(code) {
 }
 
 describe('css', () => {
+	before(() => {
+		setupHtmlEqual();
+	});
+
 	fs.readdirSync(`${__dirname}/samples`).forEach(dir => {
 		if (dir[0] === '.') return;
 
@@ -80,7 +84,17 @@ describe('css', () => {
 				css: read(`${__dirname}/samples/${dir}/expected.css`)
 			};
 
-			assert.equal(dom.css.code.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz'), expected.css);
+			const actual_css = dom.css.code.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz');
+			try {
+				assert.equal(actual_css, expected.css);
+			} catch (error) {
+				if (shouldUpdateExpected()) {
+					fs.writeFileSync(`${__dirname}/samples/${dir}/expected.css`, actual_css);
+					console.log(`Updated ${dir}/expected.css.`);
+				} else {
+					throw error;
+				}
+			}
 
 			let ClientComponent;
 			let ServerComponent;
@@ -114,10 +128,8 @@ describe('css', () => {
 
 					fs.writeFileSync(`${__dirname}/samples/${dir}/_actual.html`, html);
 
-					assert.equal(
-						normalizeHtml(window, html.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz')),
-						normalizeHtml(window, expected.html)
-					);
+					const actual_html = html.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz');
+					assert.htmlEqual(actual_html, expected.html);
 
 					window.document.head.innerHTML = ''; // remove added styles
 				} catch (err) {
@@ -127,13 +139,8 @@ describe('css', () => {
 
 				// ssr
 				try {
-					assert.equal(
-						normalizeHtml(
-							window,
-							ServerComponent.render(config.props).html.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz')
-						),
-						normalizeHtml(window, expected.html)
-					);
+					const actual_ssr = ServerComponent.render(config.props).html.replace(/svelte(-ref)?-[a-z0-9]+/g, (m, $1) => $1 ? m : 'svelte-xyz');
+					assert.htmlEqual(actual_ssr, expected.html);
 				} catch (err) {
 					console.log(ssr.js.code);
 					throw err;
diff --git a/test/helpers.js b/test/helpers.js
index d4f507fc87..ff40ac5f79 100644
--- a/test/helpers.js
+++ b/test/helpers.js
@@ -206,6 +206,10 @@ export function showOutput(cwd, options = {}, compile = svelte.compile) {
 	});
 }
 
+export function shouldUpdateExpected() {
+	return process.argv.includes('--update');
+}
+
 export function spaces(i) {
 	let result = '';
 	while (i--) result += ' ';
diff --git a/test/hydration/index.js b/test/hydration/index.js
index 85e2ccc4a0..a0bfd6de4b 100644
--- a/test/hydration/index.js
+++ b/test/hydration/index.js
@@ -7,7 +7,8 @@ import {
 	loadConfig,
 	loadSvelte,
 	env,
-	setupHtmlEqual
+	setupHtmlEqual,
+	shouldUpdateExpected
 } from '../helpers.js';
 
 let compileOptions = null;
@@ -76,7 +77,16 @@ describe('hydration', () => {
 					props: config.props
 				});
 
-				assert.htmlEqual(target.innerHTML, fs.readFileSync(`${cwd}/_after.html`, 'utf-8'));
+				try {
+					assert.htmlEqual(target.innerHTML, fs.readFileSync(`${cwd}/_after.html`, 'utf-8'));
+				} catch (error) {
+					if (shouldUpdateExpected()) {
+						fs.writeFileSync(`${cwd}/_after.html`, target.innerHTML);
+						console.log(`Updated ${cwd}/_after.html.`);
+					} else {
+						throw error;
+					}
+				}
 
 				if (config.test) {
 					config.test(assert, target, snapshot, component, window);
diff --git a/test/js/index.js b/test/js/index.js
index 5fd632d606..799be215f2 100644
--- a/test/js/index.js
+++ b/test/js/index.js
@@ -1,7 +1,8 @@
 import * as assert from "assert";
 import * as fs from "fs";
 import * as path from "path";
-import { loadConfig, svelte } from "../helpers.js";
+import * as colors from "kleur";
+import { loadConfig, svelte, shouldUpdateExpected } from "../helpers.js";
 
 describe("js", () => {
 	fs.readdirSync(`${__dirname}/samples`).forEach(dir => {
@@ -14,8 +15,14 @@ describe("js", () => {
 			throw new Error("Forgot to remove `solo: true` from test");
 		}
 
+		dir = path.resolve(`${__dirname}/samples`, dir);
+
+		if (!fs.existsSync(`${dir}/input.svelte`)) {
+			console.log(colors.red().bold(`Missing file ${dir}/input.svelte. If you recently switched branches you may need to delete this directory`));
+			return;
+		}
+
 		(solo ? it.only : it)(dir, () => {
-			dir = path.resolve(`${__dirname}/samples`, dir);
 			const config = loadConfig(`${dir}/_config.js`);
 
 			const input = fs.readFileSync(`${dir}/input.svelte`, "utf-8").replace(/\s+$/, "");
@@ -34,12 +41,32 @@ describe("js", () => {
 			const output = `${dir}/_actual.js`;
 			fs.writeFileSync(output, actual);
 
-			const expected = fs.readFileSync(`${dir}/expected.js`, "utf-8");
+			const expectedPath = `${dir}/expected.js`;
+
+			let expected = '';
+			try {
+				expected = fs.readFileSync(expectedPath, "utf-8");
+			} catch (error) {
+				console.log(error);
+				if (error.code === 'ENOENT') {
+					// missing expected.js
+					fs.writeFileSync(expectedPath, actual);
+				}
+			}
 
-			assert.equal(
-				actual.trim().replace(/^[ \t]+$/gm, ""),
-				expected.trim().replace(/^[ \t]+$/gm, "")
-			);
+			try {
+				assert.equal(
+					actual.trim().replace(/^[ \t]+$/gm, ""),
+					expected.trim().replace(/^[ \t]+$/gm, "")
+				);
+			} catch (error) {
+				if (shouldUpdateExpected()) {
+					fs.writeFileSync(expectedPath, actual);
+					console.log(`Updated ${expectedPath}.`);
+				} else {
+					throw error;
+				}
+			}
 		});
 	});
 });
diff --git a/test/parser/index.js b/test/parser/index.js
index 0188fac431..27c5ec1563 100644
--- a/test/parser/index.js
+++ b/test/parser/index.js
@@ -1,6 +1,6 @@
 import * as assert from 'assert';
 import * as fs from 'fs';
-import { svelte, tryToLoadJson } from '../helpers.js';
+import { svelte, tryToLoadJson, shouldUpdateExpected } from '../helpers.js';
 
 describe('parse', () => {
 	fs.readdirSync(`${__dirname}/samples`).forEach(dir => {
@@ -15,7 +15,9 @@ describe('parse', () => {
 			);
 		}
 
-		(solo ? it.only : it)(dir, () => {
+		const skip = !fs.existsSync(`${__dirname}/samples/${dir}/input.svelte`);
+
+		(skip ? it.skip : solo ? it.only : it)(dir, () => {
 			const options = tryToLoadJson(`${__dirname}/samples/${dir}/options.json`) || {};
 
 			const input = fs.readFileSync(`${__dirname}/samples/${dir}/input.svelte`, 'utf-8').replace(/\s+$/, '');
diff --git a/test/preprocess/samples/comments/_config.js b/test/preprocess/samples/comments/_config.js
new file mode 100644
index 0000000000..4416d121d0
--- /dev/null
+++ b/test/preprocess/samples/comments/_config.js
@@ -0,0 +1,8 @@
+export default {
+	preprocess: [
+		{
+			script: ({ content }) => ({ code: content.replace(/one/g, 'two') }),
+			style: ({ content }) => ({ code: content.replace(/one/g, 'three') }),
+		},
+	],
+};
diff --git a/test/preprocess/samples/comments/input.svelte b/test/preprocess/samples/comments/input.svelte
new file mode 100644
index 0000000000..1a01971f7f
--- /dev/null
+++ b/test/preprocess/samples/comments/input.svelte
@@ -0,0 +1,25 @@
+<style>
+	one
+</style>
+
+<script>
+	one
+</script>
+
+<!-- <style>
+	one
+</style> -->
+
+<!-- <script>
+	one
+</script> -->
+
+<style>
+<!-- one -->
+</style>
+
+<script>
+<!-- one -->
+</script>
+
+
diff --git a/test/preprocess/samples/comments/output.svelte b/test/preprocess/samples/comments/output.svelte
new file mode 100644
index 0000000000..2d3538af17
--- /dev/null
+++ b/test/preprocess/samples/comments/output.svelte
@@ -0,0 +1,25 @@
+<style>
+	three
+</style>
+
+<script>
+	two
+</script>
+
+<!-- <style>
+	one
+</style> -->
+
+<!-- <script>
+	one
+</script> -->
+
+<style>
+<!-- three -->
+</style>
+
+<script>
+<!-- two -->
+</script>
+
+
diff --git a/test/server-side-rendering/index.js b/test/server-side-rendering/index.js
index 73b286044f..768917e833 100644
--- a/test/server-side-rendering/index.js
+++ b/test/server-side-rendering/index.js
@@ -6,7 +6,8 @@ import {
 	showOutput,
 	loadConfig,
 	setupHtmlEqual,
-	tryToLoadJson
+	tryToLoadJson,
+	shouldUpdateExpected
 } from "../helpers.js";
 
 function tryToReadFile(file) {
@@ -58,18 +59,47 @@ describe("ssr", () => {
 				fs.writeFileSync(`${dir}/_actual.html`, html);
 				if (css.code) fs.writeFileSync(`${dir}/_actual.css`, css.code);
 
-				assert.htmlEqual(html, expectedHtml);
-				assert.equal(
-					css.code.replace(/^\s+/gm, ""),
-					expectedCss.replace(/^\s+/gm, "")
-				);
+				try {
+					assert.htmlEqual(html, expectedHtml);
+				} catch (error) {
+					if (shouldUpdateExpected()) {
+						fs.writeFileSync(`${dir}/_expected.html`, html);
+						console.log(`Updated ${dir}/_expected.html.`);
+					} else {
+						throw error;
+					}
+				}
+
+				try {
+					assert.equal(
+						css.code.replace(/^\s+/gm, ""),
+						expectedCss.replace(/^\s+/gm, "")
+					);
+				} catch (error) {
+					if (shouldUpdateExpected()) {
+						fs.writeFileSync(`${dir}/_expected.css`, css.code);
+						console.log(`Updated ${dir}/_expected.css.`);
+					} else {
+						throw error;
+					}
+				}
 
 				if (fs.existsSync(`${dir}/_expected-head.html`)) {
 					fs.writeFileSync(`${dir}/_actual-head.html`, head);
-					assert.htmlEqual(
-						head,
-						fs.readFileSync(`${dir}/_expected-head.html`, 'utf-8')
-					);
+
+					try {
+						assert.htmlEqual(
+							head,
+							fs.readFileSync(`${dir}/_expected-head.html`, 'utf-8')
+						);
+					} catch (error) {
+						if (shouldUpdateExpected()) {
+							fs.writeFileSync(`${dir}/_expected-head.html`, head);
+							console.log(`Updated ${dir}/_expected-head.html.`);
+						} else {
+							throw error;
+						}
+					}
 				}
 
 				if (show) showOutput(dir, { generate: 'ssr', format: 'cjs' });