Make dart2js code retain the original stack trace for uncaught async errors.

sdk/lib/_internal/js_runtime/lib/async_patch.dart

 // Copyright (c) 2012, 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.
 
 // Patch file for the dart:async library.
 
 import 'dart:_js_helper' show
     patch,
     ExceptionAndStackTrace,
     Primitives,
     convertDartClosureToJS,
     getTraceFromException,
     requiresPreamble,
+    wrapException,
     unwrapException;
 import 'dart:_isolate_helper' show
     IsolateNatives,
     TimerImpl,
     leaveJsAsync,
     enterJsAsync,
     isWorker;
 
 import 'dart:_foreign_helper' show JS;
 
@@ skipping 478 matching lines @@
   // Each invocation should give a body with fresh state.
   final dynamic /* js function */ _outerHelper;
 
   _SyncStarIterable(this._outerHelper);
 
   Iterator get iterator => new _SyncStarIterator(JS('', '#()', _outerHelper));
 }
 
 @patch
 void _rethrow(Object error, StackTrace stackTrace) {
-  throw new AsyncError(error, stackTrace);
+  error = wrapException(error);
+  JS("void", "#.stack = #", error, stackTrace.toString());

[Siggi Cherem (dart-lang), 2015/10/05 17:09:33]

I'm not super familiar with this logic yet, but I wonder if we need to do this
assignment of the stack, or if we can only set the cachedTrace property like we
did with the sync rethrow?
(https://github.com/dart-lang/sdk/blob/8697392367be57f9f595abca0cd106c2f3e56f3...)

One of my concerns on setting stack is that I recall IE sets the stack after the
throw, so I'm not sure if it will override our stack in that case?

[Lasse Reichstein Nielsen, 2015/10/06 07:57:26]

As I understand it, the "generally accepted" behavior is that if you actively
assign to the stack property, it's not automatically overwritten after that.

This seems to work the same in Chrome, Firefox and IE 11 (I can't test in Safari
or earlier IEs).

Test:
  function test() {
    var e = new Error();
    e.stack = "Arglebargle glop glyf";
    throw e;
  }
  test();
Then check the stack of the throw error. 

+  JS("void", "throw #", error);
 }