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 generates code like this, but
+        // only when it is known that the 'expr' throws, and the return is just
+        // to tell the JavaScript VM that the code won't continue here.
+        // It is therefore interpreted as `expr; return;`
+        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;
   }
 }