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

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library rewrite_async;
 
 import "dart:math" show max;
 import 'dart:collection';
 
 import 'package:js_runtime/shared/async_await_error_codes.dart'
@@ skipping 1263 matching lines @@
     return withExpression(
         node.value, (js.Expression value) => new js.Property(node.name, value),
         store: false);
   }
 
   @override
   js.Expression visitRegExpLiteral(js.RegExpLiteral node) => node;
 
   @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);
   }
 
   @override
   void visitSwitch(js.Switch node) {
     if (!node.cases.any(shouldTransform)) {
       // If only the key has an await, translation can be simplified.
       bool oldInsideUntranslated = insideUntranslatedBreakable;
       insideUntranslatedBreakable = true;
@@ skipping 1277 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }