dart2js: fix mixin support in deferred libraries.

tests/language/deferred_mixin_lib1.dart

+// Copyright (c) 2014, 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 lib1;
+
+import "deferred_mixin_shared.dart";
+import "deferred_mixin_test.dart";
+
+class Mixin {
+  foo() => "lib1.Mixin";
+}
+
+class A extends Object with NonDeferredMixin {
+  foo() {
+    return "A with " + super.foo();
+  }
+}
+
+class B extends Object with Mixin {
+  foo() {
+    return "B with " + super.foo();
+  }
+}
+
+class C extends Object with Mixin, NonDeferredMixin1 {
+  foo() {
+    return "C with " + super.foo();
+  }
+}
+
+class D extends Object with NonDeferredMixin2, Mixin {
+  foo() {
+    return "D with " + super.foo();
+  }
+}
+
+class E extends Object with SharedMixin {
+  foo() {
+    return "E with " + super.foo();
+  }
+}