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

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

Index: sdk/lib/_internal/js_runtime/lib/async_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/async_patch.dart b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
index 1155e79f76964886ed61ab0c2006b9ed1c5e292f..450e30b434f693ac6a32b88f95bfb8c45ada91aa 100644
--- a/sdk/lib/_internal/js_runtime/lib/async_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/async_patch.dart
@@ -11,6 +11,7 @@ import 'dart:_js_helper' show
     convertDartClosureToJS,
     getTraceFromException,
     requiresPreamble,
+    wrapException,
     unwrapException;
 import 'dart:_isolate_helper' show
     IsolateNatives,
@@ -509,5 +510,7 @@ class _SyncStarIterable extends IterableBase {
 
 @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);
 }