add checks needed for covariant generics, and List<E>

test/codegen/covariance.dart

+// Copyright (c) 2015, 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.
+
+class Foo<T> {
+  T _t;
+  add(T t) {
+    _t = t;
+  }
+
+  forEach(void fn(T t)) {
+    // No check needed for `fn`
+    fn(_t);
+  }
+}
+
+class Bar extends Foo<int> {
+  add(int x) {
+    print('Bar.add got $x');
+    super.add(x);
+  }
+}
+
+main() {
+  Foo<Object> foo = new Bar();
+  foo.add('hi'); // should throw
+}