Turn error into warning for non-static member access through the class.

tests/language/mixin_getter_regression_test.dart

+// Copyright (c) 2013, 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 case for dart2js bug where the getter for y wasn't
+// properly mixed in.
+
+class C {
+  int x;
+  int get y => x;
+}
+
+class E {
+  int z = 10;
+}
+
+class D extends E with C {
+  int w = 42;
+}
+
+main() {
+  var d = new D();
+  d.x = 37;
+  Expect.equals(37, d.x);
+  Expect.equals(10, d.z);
+  Expect.equals(42, d.w);
+  Expect.equals(37, d.y);
+}