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

pkg/fletchc/lib/src/fletch_compiler_hack.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.
+
+part of fletchc.fletch_compiler;
+
+abstract class FletchCompilerHack extends apiimpl.Compiler {
+  FletchCompilerHack(
+      api.CompilerInputProvider provider,
+      api.CompilerOutputProvider outputProvider,
+      api.DiagnosticHandler handler,
+      Uri libraryRoot,
+      Uri packageRoot,
+      List<String> options,
+      Map<String, dynamic> environment)
+      : super(provider, outputProvider, handler, libraryRoot, packageRoot,
+              options, environment) {
+    switchBackendHack();
+  }
+
+  void switchBackendHack() {
+    // TODO(ahe): Modify dart2js to support a custom backend directly, and
+    // remove this method.
+    int backendTaskCount = backend.tasks.length;
+    int apiimplTaskCount = 2;
+    int baseTaskCount = tasks.length - backendTaskCount - apiimplTaskCount;
+
+    tasks.removeRange(baseTaskCount, baseTaskCount + backendTaskCount);
+
+    backend = new FletchBackend(this);
+    tasks.addAll(backend.tasks);
+  }
+}