Allow `return <expr>` in sync* and async* javascript.

pkg/compiler/lib/src/js/rewrite_async.dart

Index: pkg/compiler/lib/src/js/rewrite_async.dart
diff --git a/pkg/compiler/lib/src/js/rewrite_async.dart b/pkg/compiler/lib/src/js/rewrite_async.dart
index 41a84661af3be219797ab4975d6421f025ff78da..27c001b6d52b096c9c3a013a8ef740ec6bcedbfd 100644
--- a/pkg/compiler/lib/src/js/rewrite_async.dart
+++ b/pkg/compiler/lib/src/js/rewrite_async.dart
@@ -1281,12 +1281,18 @@ abstract class AsyncRewriterBase extends js.NodeVisitor {
 
   @override
   void visitReturn(js.Return node) {
-    assert(node.value == null || (!isSyncStar && !isAsyncStar));
     js.Node target = analysis.targets[node];
     if (node.value != null) {
-      withExpression(node.value, (js.Expression value) {
-        addStatement(js.js.statement("# = #;", [returnValue, value]));
-      }, store: false);
+      if(isSyncStar || isAsyncStar) {
+        // Even though `return expr;` is not allowed in the dart sync* and
+        // async*  code, the backend sometimes generated code like this.
+        // It is interpreted as `expr; return;`

[floitsch, 2015/07/15 14:03:16]

Add more comment of why this could ever be legal. (I.e. we know that the 'expr'
throws, and the return is just to tell V8 that the code won't continue here).

+        visitExpressionIgnoreResult(node.value);
+      } else {
+        withExpression(node.value, (js.Expression value) {
+          addStatement(js.js.statement("# = #;", [returnValue, value]));
+        }, store: false);
+      }
     }
     translateJump(target, exitLabel);
   }