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

tests/lib/async/dart2js_uncaught_error_test.dart

Index: tests/lib/async/dart2js_uncaught_error_test.dart
diff --git a/tests/lib/async/dart2js_uncaught_error_test.dart b/tests/lib/async/dart2js_uncaught_error_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..6c7f12d7e1068a29bde3baf38a01ea070cde2347
--- /dev/null
+++ b/tests/lib/async/dart2js_uncaught_error_test.dart
@@ -0,0 +1,46 @@
+// 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.
+
+import "dart:async";
+import "dart:js";
+import "dart:collection" show Queue;
+import "package:expect/expect.dart";
+import "package:async_helper/async_helper.dart";
+
+var errors = new Queue();
+int ctr = 0;
+
+main() {
+  print("STARTED");
+  asyncStart();
+
+  void errorHandler(self, message, url, line, [column, error]) {
+    print(">> $message / $ctr");
+    var expect = errors.removeFirst();
+    if (ctr == 2) {
+      asyncEnd();
+      print("DONE");
+    }
+    Expect.equals(expect[0].toString(), message);
+    Expect.equals(expect[1].toString(), error["stack"].toString());
+  }
+  context["onerror"] = new JsFunction.withThis(errorHandler);
+
+  void throwit() {
+    var err = ++ctr;
+    try {
+      throw err;
+    } catch (e, s) {
+      errors.add([e, s]);
+      rethrow;
+    }
+  }
+
+  () async {
+    () async {
+      throwit();
+    }();
+    throwit();
+  }();
+}