Add tests for generic methods in strong mode

tests/language_strong/generic_methods_simple_is_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.
+
+// Test that type parameters in generic methods can be used in is-expressions.
+
+library generic_methods_simple_is_expression_test;
+
+import "package:expect/expect.dart";
+
+bool fun<T>(int n) {
+  return n is T;
+}
+
+main() {
+  Expect.isTrue(fun<int>(42));
+  Expect.isFalse(fun<String>(42));
+}