Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/tree/nodes.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 tree;
+import '../common.dart';
+import '../tokens/precedence_constants.dart' as Precedence show FUNCTION_INFO;
+import '../tokens/token.dart' show BeginGroupToken, Token;
+import '../tokens/token_constants.dart' as Tokens show IDENTIFIER_TOKEN, KEYWORD_TOKEN, PLUS_TOKEN;
+import '../util/util.dart';
+import '../util/characters.dart';
+import '../resolution/secret_tree_element.dart' show NullTreeElementMixin, StoredTreeElementMixin;
+import '../elements/elements.dart' show MetadataAnnotation;
+import 'dartstring.dart';
+import 'prettyprint.dart';
+import 'unparser.dart';
 
 abstract class Visitor<R> {
   const Visitor();
 
   R visitNode(Node node);
 
   R visitAssert(Assert node) => visitStatement(node);
   R visitAsyncForIn(AsyncForIn node) => visitLoop(node);
   R visitAsyncModifier(AsyncModifier node) => visitNode(node);
   R visitAwait(Await node) => visitExpression(node);
@@ skipping 2465 matching lines @@
 
   // VariableDefinitions.
   get metadata => null;
   get type => null;
 
   // Typedef.
   get typeParameters => null;
   get formals => null;
   get typedefKeyword => null;
 }