Add tests for generic methods in strong mode

tests/language_strong/generic_methods_simple_as_expression_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.
+
+library generic_methods_simple_as_expression_test;
+
+import "package:expect/expect.dart";
+
+T cast<T>(dynamic obj) {
+  return obj as T;
+}
+
+main() {
+  Expect.isTrue(cast<num>(42) == 42);
+  Expect.throws(() => cast<String>(42), (e) => e is CastError);
+}