dart2js: Bugfix in async rewriter preanalysis.

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 2427 matching lines @@
     return unsupported(node);
   }
 
   @override
   bool visitLiteralString(js.LiteralString node) {
     return false;
   }
 
   @override
   bool visitStringConcatenation(js.StringConcatenation node) {
-    return true;
+    return false;
   }
 
   @override
   bool visitName(js.Name node) {
-    return true;
+    return false;
   }
 
   @override
   bool visitNamedFunction(js.NamedFunction node) {
     return false;
   }
 
   @override
   bool visitNew(js.New node) {
     return visitCall(node);
@@ skipping 117 matching lines @@
     return condition || body;
   }
 
   @override
   bool visitDartYield(js.DartYield node) {
     hasYield = true;
     visit(node.expression);
     return true;
   }
 }