Fix crash when deferred loading would encounter an unresolved super setter.

tests/language/deferred_super_dependency_test.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.
+
+// Regression test.
+// lib.C.foo has code that references `super.foo=` that does not exist. This
+// used to cause a crash.
+
+import "package:expect/expect.dart";
+import "deferred_super_dependency_lib.dart" deferred as lib; /// 01: static type warning
+
+main() async {
+  await lib.loadLibrary(); /// 01: continued
+  Expect.throws(() => new lib.C().foo(), (e) => e is NoSuchMethodError); /// 01: continued
+}