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

pkg/compiler/lib/src/tree/tree.dart

Index: pkg/compiler/lib/src/tree/tree.dart
diff --git a/pkg/compiler/lib/src/tree/tree.dart b/pkg/compiler/lib/src/tree/tree.dart
index 8c65fabdd51382f7f30cad9d263aa03a108d8d9c..71e9b6d4ebb40b6637f3dec0aaf956608386df6b 100644
--- a/pkg/compiler/lib/src/tree/tree.dart
+++ b/pkg/compiler/lib/src/tree/tree.dart
@@ -4,29 +4,7 @@
 
 library tree;
 
-import 'dart:collection';
-
-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;
-
-part 'dartstring.dart';
-part 'nodes.dart';
-part 'prettyprint.dart';
-part 'unparser.dart';
+export 'dartstring.dart';
+export 'nodes.dart';
+export 'prettyprint.dart';
+export 'unparser.dart';