mount yield blocks immediately, if yield has a parent node (fixes #561)

Author: Rich-Harris

src/generators/dom/visitors/YieldTag.js

@@ -1,9 +1,11 @@
 export default function visitYieldTag ( generator, block, state ) {
-	block.builders.mount.addLine(
-		`${block.component}._yield && ${block.component}._yield.mount( ${state.parentNode || block.target}, null );`
+	const parentNode = state.parentNode || block.target;
+
+	( state.parentNode ? block.builders.create : block.builders.mount ).addLine(
+		`if ( ${block.component}._yield ) ${block.component}._yield.mount( ${parentNode}, null );`
 	);
 
 	block.builders.destroy.addLine(
-		`${block.component}._yield && ${block.component}._yield.destroy( detach );`
+		`if ( ${block.component}._yield ) ${block.component}._yield.destroy( detach );`
 	);
 }