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

pkg/fletchc/lib/src/fletch_backend.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_backend;
+
+import 'package:compiler/src/dart2jslib.dart' show
+    Backend,
+    BackendConstantEnvironment,
+    CodegenWorkItem,
+    ConstantCompilerTask,
+    ConstantSystem,
+    MessageKind,
+    Registry,
+    ResolutionEnqueuer;
+
+import '../bytecodes.dart' show Bytecode;
+
+import 'fletch_context.dart';
+
+class FletchBackend extends Backend {
+  final FletchContext context;
+
+  FletchBackend(FletchCompiler compiler)
+      : this.context = compiler.context,
+        super(compiler) {
+    context.resolutionCallbacks = new FletchResolutionCallbacks(context);
+  }
+
+  FletchResolutionCallbacks get resolutionCallbacks {
+    return context.resolutionCallbacks;
+  }
+
+  List<CompilerTask> get tasks => <CompilerTask>[];
+
+  ConstantSystem get constantSystem {
+    throw new UnsupportedError("get constantSystem");
+  }
+
+  BackendConstantEnvironment get constants {
+    throw new UnsupportedError("get constants");
+  }
+
+  ConstantCompilerTask get constantCompilerTask {
+    throw new UnsupportedError("get constantCompilerTask");
+  }
+
+  void enqueueHelpers(
+      ResolutionEnqueuer world,
+      Registry registry) {
+  }
+
+  void codegen(CodegenWorkItem work) {
+  }
+
+  bool get canHandleCompilationFailed => true;
+
+  int assembleProgram() {
+    compiler.reportHint(
+        compiler.mainFunction,
+        MessageKind.GENERIC,
+        {'text': 'Compiling ${compiler.mainFunction.name}'});
+
+    BytecodeBuilder builder =
+        new BytecodeBuilder(context, compiler.mainFunction);
+    compiler.mainFunction.node.accept(builder);
+    print("Constants");
+    builder.constants.forEach((constant, int index) {
+      print("  #$index: $constant");
+    });
+
+    print("Bytecodes:");
+    int offset = 0;
+    for (Bytecode bytecode in builder.bytecodes) {
+      print("  $offset: $bytecode");
+      offset += bytecode.size;
+    }
+  }
+}