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 c57b3d2be0819541eec1420696df3ba552fed7a8..4d91d7948c31ceb37b515b2a04f3641619b5f5cb 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,
@@ -518,5 +519,7 @@ 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());
+  JS("void", "throw #", error);
 }