[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;
+
+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));
+  });
+}