Use correct compilation unit for inlined field initializers.

tests/language/inline_super_field_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.
+
+// Test that source maps use the correct compilation unit when super class
+// fields from another compilation unit are inlined.
+
+library inline_super_field_test;
+
+import "inline_super_field_lib.dart";
+
+class S {
+}
+
+class C extends S with M1 { }
+
+void main() {
+  var c = new C();
+  Expect.equals(1, c.bar);
+}