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

pkg/fletchc/lib/src/fletch_context.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_context;
+
+import 'fletch_compiler.dart' show
+    FletchCompiler;
+
+export 'fletch_compiler.dart' show
+    FletchCompiler;
+
+import 'fletch_backend.dart' show
+    FletchBackend;
+
+export 'fletch_backend.dart' show
+    FletchBackend;
+
+import 'fletch_resolution_callbacks.dart' show
+    FletchResolutionCallbacks;
+
+export 'fletch_resolution_callbacks.dart' show
+    FletchResolutionCallbacks;
+
+export 'bytecode_builder.dart' show
+    BytecodeBuilder;
+
+class FletchContext {
+  final FletchCompiler compiler;
+
+  FletchResolutionCallbacks resolutionCallbacks;
+
+  FletchContext(this.compiler);
+
+  FletchBackend get backend => compiler.backend;
+}