implement null aware ops, fixes #249

test/codegen/language/conditional_access_helper.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.
+
+// Library used by conditional_property_assignment_test.dart,
+// conditional_property_access_test.dart, and
+// conditional_method_invocation_test.dart, all of which import it using the
+// prefix "h.".
+
+library lib;
+
+var topLevelVar;
+
+void topLevelFunction() {}
+
+class C {
+  static int staticInt;
+  static staticF(callback()) => callback();
+  static int staticG(int callback()) => callback();
+}
+
+C nullC() => null;
+
+class D {
+  static E staticE;
+}
+
+class E {
+  G operator+(int i) => new I();
+  G operator-(int i) => new I();
+}
+
+class F {}
+
+class G extends E implements F {}
+
+class H {}
+
+class I extends G implements H {}