Add some tests for calls with checks in strong mode.

tests/language_strong/covariant_subtyping_unsafe_call3_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 'package:expect/expect.dart';
+
+class Implementation {
+  dynamic method(int x) {}
+}
+
+abstract class Interface1<T> {
+  dynamic method(T x);
+}
+
+abstract class Interface2<T> {
+  dynamic method(T x);
+}
+
+class Subclass extends Implementation
+    implements Interface1<int>, Interface2<int> {}
+
+main() {
+  Subclass subclass = new Subclass();
+
+  Interface1<int> intInterface1 = subclass;
+  Interface1<num> numInterface1 = intInterface1;
+  Expect.throws(() => numInterface1.method(2.5));
+
+  Interface2<int> intInterface2 = subclass;
+  Interface2<num> numInterface2 = intInterface2;
+  Expect.throws(() => numInterface2.method(2.5));
+}