Begin writing tests of the runtime checks needed for Dart 2.0 soundness.

pkg/front_end/testcases/runtime_checks/covariant_setter.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.
+
+/*@testedFeatures=checks*/
+library test;
+
+typedef void F<T>(T x);
+
+class C<T> {
+  T /*@checkFormal=semiSafe*/ x;
+  void f(T /*@checkFormal=semiSafe*/ value) {
+    this.x = value;
+  }
+}
+
+void g(C<num> c) {
+  c. /*@checkCall=interface(semiTyped:0)*/ x = 1.5;
+}
+
+void main() {}