Ensure that only one {{yield}} can be used

compiler/generate/index.js

@@ -490,7 +490,7 @@ export default function generate ( parsed, source, options ) {
 			};
 
 			this.appendChild = function appendChild ( child ) {
-				this.yield.appendChild(child)
+				this.yield.appendChild(child);
 			}
 
 			${initStatements.join( '\n\n' )}

compiler/generate/visitors/MustacheTag.js

@@ -8,9 +8,14 @@ export default {
 		generator.addSourcemapLocations( node.expression );
 
 		if (node.expression.name === 'yield') {
+			if (generator.hasYield === undefined) {
+				generator.hasYield	= true;
 				generator.current.initStatements.push( deindent`
 					component.yield = ${generator.current.target};
 				` );
+			} else {
+				throw new Error( `Only one {{yield}} per component.` );
+			}
 		} else {
 			generator.addElement( name, `document.createTextNode( ${snippet} )`, true );
 

test/compiler/component-yield-one/_config.js

@@ -0,0 +1,5 @@
+export default {
+	compileError: function ( assert, err ) {
+		assert.equal('Error: Only one {{yield}} per component.', err)
+	}
+};

test/compiler/component-yield-one/main.html

@@ -0,0 +1,2 @@
+<p>{{yield}}</p>
+<p>{{yield}}</p>

test/test.js

@@ -228,7 +228,7 @@ describe( 'svelte', () => {
 					compiled = svelte.compile( source );
 				} catch ( err ) {
 					if ( config.compileError ) {
-						config.compileError( err );
+						config.compileError( assert, err );
 						return;
 					} else {
 						throw err;