Ensure stack trace are identical.

dart/sdk/lib/_internal/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:collection';
 import 'dart:_foreign_helper' show DART_CLOSURE_TO_JS,
                                    JS,
                                    JS_CALL_IN_ISOLATE,
@@ skipping 1402 matching lines @@
   // Just return the exception. We should not wrap it because in case
   // the exception comes from the DOM, it is a JavaScript
   // object backed by a native Dart class.
   return ex;
 }
 
 /**
  * Called by generated code to fetch the stack trace from an
  * exception. Should never return null.
  */
-StackTrace getTraceFromException(exception) => new _StackTrace(exception);
+StackTrace getTraceFromException(exception) {
+  if (exception == null) return new _StackTrace(exception);
+  _StackTrace trace = JS('_StackTrace|Null', r'#.$cachedTrace', exception);
+  if (trace != null) return trace;
+  trace = new _StackTrace(exception);
+  return JS('_StackTrace', r'#.$cachedTrace = #', exception, trace);
+}
 
 class _StackTrace implements StackTrace {
   var _exception;
   String _trace;
   _StackTrace(this._exception);
 
   String toString() {
     if (_trace != null) return _trace;
 
     String trace;
@@ skipping 578 matching lines @@
 }
 
 /**
  * Error thrown when a runtime error occurs.
  */
 class RuntimeError extends Error {
   final message;
   RuntimeError(this.message);
   String toString() => "RuntimeError: $message";
 }