Tweak toString() for better type inference

sdk/lib/_internal/compiler/js_lib/js_helper.dart

 // Copyright (c) 2013, 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 _js_helper;
 
 import 'dart:_js_embedded_names' show
     GET_TYPE_FROM_NAME,
     GET_ISOLATE_TAG,
     INTERCEPTED_NAMES,
@@ skipping 1811 matching lines @@
  * exception. Should never return null.
  */
 StackTrace getTraceFromException(exception) => new _StackTrace(exception);
 
 class _StackTrace implements StackTrace {
   var _exception;
   String _trace;
   _StackTrace(this._exception);
 
   String toString() {
-    if (_trace != null) return _trace;
+    if (_trace != null) return JS('String', '#', _trace);
 
     String trace;
     if (JS('bool', 'typeof # === "object"', _exception)) {
       trace = JS("String|Null", r"#.stack", _exception);
     }
     return _trace = (trace == null) ? '' : trace;
   }
 }
 
 int objectHashCode(var object) {
@@ skipping 1612 matching lines @@
   throw new MainError("No top-level function named 'main'.");
 }
 
 void badMain() {
   throw new MainError("'main' is not a function.");
 }
 
 void mainHasTooManyParameters() {
   throw new MainError("'main' expects too many parameters.");
 }