Refactor implementation to address review comments and API design suggestions from Luke.

pkg/fletchc/lib/src/fletch_resolution_callbacks.dart

+// Copyright (c) 2015, the Fletch 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.md file.
+
+library fletchc.fletch_resolution_callbacks;
+
+import 'package:compiler/src/dart2jslib.dart' show
+    ResolutionCallbacks;
+
+import 'fletch_context.dart' show
+    FletchContext;
+
+class FletchResolutionCallbacks extends ResolutionCallbacks {
+  final FletchContext context;
+
+  FletchResolutionCallbacks(this.context);
+}