Refactor and simplify the dart2dart renamer.

sdk/lib/_internal/compiler/implementation/dart_backend/emitter.dart

-// Copyright (c) 2012, 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 dart_backend;
-
-void emitCode(
-      Unparser unparser,
-      Map<LibraryElement, String> imports,
-      Iterable<Node> topLevelNodes,
-      Map<ClassNode, Iterable<Node>> classMembers) {
-  imports.forEach((libraryElement, prefix) {
-    unparser.unparseImportTag('${libraryElement.canonicalUri}', prefix);
-  });
-
-  for (final node in topLevelNodes) {
-    if (node is ClassNode) {
-      // TODO(smok): Filter out default constructors here.
-      unparser.unparseClassWithBody(node, classMembers[node]);
-    } else {
-      unparser.unparse(node);
-    }
-    unparser.newline();
-  }
-}