Track async causal stack traces

tests/language/vm/causal_async_exception_stack_test.dart

+// Copyright (c) 2016, 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 'package:unittest/unittest.dart';
+
+thrower() async {
+  throw 'oops';
+}
+
+number() async {
+  return 4;
+}
+
+generator() async* {
+  yield await number();
+  yield await thrower();
+}
+
+foo() async {
+  await for (var i in generator()) {
+    print(i);
+  }
+}
+
+main() async {
+  try {
+    await foo();
+  } catch (e, st) {
+    expect(st.toString(), stringContainsInOrder([
+        'thrower.<thrower_async_body>',
+        '<asynchronous suspension>',
+        'thrower',
+        'generator.<generator_async_gen_body>',
+        '<asynchronous suspension>',
+        'generator',
+        'foo.<foo_async_body>',
+        '<asynchronous suspension>',
+        ]));
+  }
+}