Report wrong async function result type as type error in checked mode (Fixes #26133).

tests/language/regress_26133_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 '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;
+}
+
+main() {
+  foo().then(
+    (_) {
+      Expect.isFalse(isCheckedMode());
+    },
+    onError: (e) {
+      Expect.isTrue(isCheckedMode() && (e is TypeError));
+    }
+  );
+}
+