Adds class directive shortcut

When no expression is used in a class directive the class name will be used to evaluate whether the class should be added/removed.

E.g. the following will add the class "active" when you call `component.set({ active });`.

```html
<div class:active></div>
```

src/compile/nodes/Element.ts

@@ -855,8 +855,8 @@ export default class Element extends Node {
 			const { expression } = action;
 			let snippet, dependencies;
 			if (expression) {
-				snippet = action.expression.snippet;
-				dependencies = action.expression.dependencies;
+				snippet = expression.snippet;
+				dependencies = expression.dependencies;
 			}
 
 			const name = block.getUniqueName(
@@ -889,7 +889,16 @@ export default class Element extends Node {
 
 	addClasses(block: Block) {
 		this.classes.forEach(classDir => {
-			const { expression: { snippet, dependencies}, name } = classDir;
+			const { expression, name } = classDir;
+			let snippet, dependencies;
+			if (expression) {
+				snippet = expression.snippet;
+				dependencies = expression.dependencies;
+			} else {
+				const varName = camelCase(name);
+				snippet = `ctx.${varName}`;
+				dependencies = [varName];
+			}
 			const updater = `@toggleClass(${this.var}, "${name}", ${snippet});`;
 
 			block.builders.hydrate.addLine(updater);
@@ -978,7 +987,8 @@ export default class Element extends Node {
 		}
 
 		const classExpr = this.classes.map((classDir: Class) => {
-			const { expression: { snippet }, name } = classDir;
+			const { expression, name } = classDir;
+			const snippet = expression ? expression.snippet : `ctx.${camelCase(name)}`;
 			return `${snippet} ? "${name}" : ""`;
 		}).join(', ');
 
@@ -1026,7 +1036,7 @@ export default class Element extends Node {
 					openingTag += '${' + attribute.chunks[0].snippet + ' ? " ' + attribute.name + '" : "" }';
 				} else if (attribute.name === 'class' && classExpr) {
 					addClassAttribute = false;
-					openingTag += ` class="\${ [\`${attribute.stringifyForSsr()}\`, ${classExpr} ].join(' ') }"`;
+					openingTag += ` class="\${ [\`${attribute.stringifyForSsr()}\`, ${classExpr} ].join(' ').trim() }"`;
 				} else {
 					openingTag += ` ${attribute.name}="${attribute.stringifyForSsr()}"`;
 				}
@@ -1034,7 +1044,7 @@ export default class Element extends Node {
 		}
 
 		if (addClassAttribute) {
-			openingTag += ` class="\${ [${classExpr}].join(' ') }"`;
+			openingTag += ` class="\${ [${classExpr}].join(' ').trim() }"`;
 		}
 
 		openingTag += '>';
@@ -1159,3 +1169,9 @@ const events = [
 			name === 'volume'
 	}
 ];
+
+function camelCase(str) {
+	return str.replace(/(-\w)/g, function (m) {
+		return m[1].toUpperCase();
+	});
+}

test/runtime/samples/class-shortcut/_config.js

@@ -0,0 +1,16 @@
+export default {
+	data: {
+		isActive: true,
+		isSelected: true,
+		myClass: 'one two'
+	},
+	html: `<div class="one two is-active isSelected"></div>`,
+
+	test ( assert, component, target, window ) {
+		component.set({ isActive: false });
+
+		assert.htmlEqual( target.innerHTML, `
+			<div class="one two isSelected"></div>
+		` );
+	}
+};

test/runtime/samples/class-shortcut/main.html

@@ -0,0 +1 @@
+<div class="{ myClass }" class:is-active class:isSelected class:not-used></div>