dart2js: add FragmentEmitter for startup-emitter.

pkg/compiler/lib/src/js_emitter/startup_emitter/fragment_emitter.dart

+// Copyright (c) 2015, 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.
+
+part of dart2js.js_emitter.startup_emitter.model_emitter;
+
+/**
+ * This class builds a JavaScript tree for a given fragment.
+ *
+ * A fragment is generally written into a separate file so that it can be
+ * loaded dynamically when a deferred library is loaded.
+ *
+ * This class is stateless and can be reused for different fragments.
+ */
+class FragmentEmitter {
+  final Compiler compiler;
+  final Namer namer;
+  final JavaScriptBackend backend;
+  final ConstantEmitter constantEmitter;
+  final ModelEmitter modelEmitter;
+
+  FragmentEmitter(this.compiler, this.namer, this.backend, this.constantEmitter,
+      this.modelEmitter);
+
+  js.Expression generateEmbeddedGlobalAccess(String global) =>
+      modelEmitter.generateEmbeddedGlobalAccess(global);
+
+  js.Expression generateConstantReference(ConstantValue value) =>
+      modelEmitter.generateConstantReference(value);
+
+  js.Statement emitMainFragment(Program program) {
+    MainFragment fragment = program.fragments.first;
+    throw new UnimplementedError('emitMain');
+  }
+
+  js.Statement emitDeferredFragment(DeferredFragment fragment,
+                                    js.Expression deferredTypes,
+                                    List<Holder> holders) {
+    throw new UnimplementedError('emitDeferred');
+  }
+}