Fix a couple of bugs with async stack traces

tests/language/vm/regress_29145_test.dart

+// Copyright (c) 2017, 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.
+// VMOptions=--optimization-counter-threshold=-1 --stacktrace-filter=completeError --stress-async-stacks
+
+// Stress test for async stack traces.
+
+import 'dart:async';
+import "package:expect/expect.dart";
+
+class A {
+  Future<List<int>> read() => new Future.error(123);
+}
+
+Future<A> haha() => new Future.microtask(() => new A());
+
+Future<List<int>> mm() async => (await haha()).read();
+
+foo() async => await mm();
+
+main() async {
+  var x;
+  try {
+    x = await foo();
+  } catch (e) {
+    Expect.equals(123, e);
+    return;
+  }
+  Expect.isTrue(false);
+}