[VM parser] Insert missing result type check in async functions using arrow

tests/language_2/regress_30339_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.
+
+import 'dart:async';
+import 'package:expect/expect.dart';
+
+isCheckedMode() {
+  try {
+    var i = 1;
+    String s = i;
+    return false;
+  } on TypeError {
+    return true;
+  }
+}
+
+var x = 'a';
+
+Future<int> foo() async {
+  return x;
+}
+
+Future<int> bar() async => x;

[cbernaschina, 2017/08/07 22:19:38]

Should we test even against possible future optimization on async/await?

By testing:

```dart
Future<String> _foo() async {
    return x;
}

Future<int> awaitFoo() async {
    return await _foo();
}
```
and

```dart
Future<String> _bar() async => x;

Future<int> awaitBar() async => await _bar();
```

[regis, 2017/08/07 22:37:38]

What kind of optimizations do you have in mind?
The type check is inserted in the ast. Any inlining will preserve this original
ast and will only remove the type test if it is correct to do so. I do not think
we need to make the test more complicated.

+
+main() {
+  foo().then((_) {
+    Expect.isFalse(isCheckedMode());
+  }, onError: (e) {
+    Expect.isTrue(isCheckedMode() && (e is TypeError));
+  });
+  bar().then((_) {
+    Expect.isFalse(isCheckedMode());
+  }, onError: (e) {
+    Expect.isTrue(isCheckedMode() && (e is TypeError));
+  });
+}