pkg/compiler: clean up imports

pkg/compiler/lib/src/resolution/tree_elements.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.
 
 library dart2js.resolution.tree_elements;
 
 import '../common.dart';
 import '../constants/expressions.dart';
 import '../dart_types.dart';
 import '../diagnostics/source_span.dart';
 import '../elements/elements.dart';
+import '../tree/tree.dart';
 import '../types/types.dart' show TypeMask;
-import '../tree/tree.dart';
+import '../universe/selector.dart' show Selector;
 import '../util/util.dart';
-import '../universe/selector.dart' show Selector;
-
 import 'secret_tree_element.dart' show getTreeElement, setTreeElement;
 import 'send_structure.dart';
 
 abstract class TreeElements {
   AnalyzableElement get analyzedElement;
   Iterable<SourceSpan> get superUses;
 
   void forEachConstantNode(f(Node n, ConstantExpression c));
 
   Element operator [](Node node);
@@ skipping 494 matching lines @@
       _nativeData = <Node, dynamic>{};
     }
     _nativeData[node] = nativeData;
   }
 
   @override
   dynamic getNativeData(Node node) {
     return _nativeData != null ? _nativeData[node] : null;
   }
 }