chore: add invariant utility (#17929)

With this, we can add invariants to the codebase so we can identify
problems like 'this batch already has roots scheduled', which indicate a
bug somewhere, without a) needing tests for scenarios that are
inherently hard to anticipate, or b) cluttering people's prod bundles

.changeset/flat-lemons-joke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+chore: add `invariant` helper for debugging

documentation/docs/98-reference/.generated/shared-errors.md

@@ -42,6 +42,12 @@ Here, `List.svelte` is using `{@render children(item)` which means it expects `P
 A snippet function was passed invalid arguments. Snippets should only be instantiated via `{@render ...}`
 ```
 
+### invariant_violation
+
+```
+An invariant violation occurred, meaning Svelte's internal assumptions were flawed. This is a bug in Svelte, not your app — please open an issue at https://github.com/sveltejs/svelte, citing the following message: "%message%"
+```
+
 ### lifecycle_outside_component
 
 ```

packages/svelte/messages/shared-errors/errors.md

@@ -34,6 +34,10 @@ Here, `List.svelte` is using `{@render children(item)` which means it expects `P
 
 > A snippet function was passed invalid arguments. Snippets should only be instantiated via `{@render ...}`
 
+## invariant_violation
+
+> An invariant violation occurred, meaning Svelte's internal assumptions were flawed. This is a bug in Svelte, not your app — please open an issue at https://github.com/sveltejs/svelte, citing the following message: "%message%"
+
 ## lifecycle_outside_component
 
 > `%name%(...)` can only be used during component initialisation

packages/svelte/src/internal/shared/dev.js

@@ -1,4 +1,6 @@
+import { DEV } from 'esm-env';
 import { define_property } from './utils.js';
+import * as e from './errors.js';
 
 /**
  * @param {string} label
@@ -63,3 +65,15 @@ export function get_stack() {
 
 	return new_lines;
 }
+
+/**
+ * @param {boolean} condition
+ * @param {string} message
+ */
+export function invariant(condition, message) {
+	if (!DEV) {
+		throw new Error('invariant(...) was not guarded by if (DEV)');
+	}
+
+	if (!condition) e.invariant_violation(message);
+}

packages/svelte/src/internal/shared/errors.js

@@ -51,6 +51,23 @@ export function invalid_snippet_arguments() {
 	}
 }
 
+/**
+ * An invariant violation occurred, meaning Svelte's internal assumptions were flawed. This is a bug in Svelte, not your app — please open an issue at https://github.com/sveltejs/svelte, citing the following message: "%message%"
+ * @param {string} message
+ * @returns {never}
+ */
+export function invariant_violation(message) {
+	if (DEV) {
+		const error = new Error(`invariant_violation\nAn invariant violation occurred, meaning Svelte's internal assumptions were flawed. This is a bug in Svelte, not your app — please open an issue at https://github.com/sveltejs/svelte, citing the following message: "${message}"\nhttps://svelte.dev/e/invariant_violation`);
+
+		error.name = 'Svelte error';
+
+		throw error;
+	} else {
+		throw new Error(`https://svelte.dev/e/invariant_violation`);
+	}
+}
+
 /**
  * `%name%(...)` can only be used during component initialisation
  * @param {string} name