Fix hints reported by analyzer.

pkg/compiler/lib/src/elements/elements.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.
 
 library elements;
 
 import '../common.dart';
 import '../common/resolution.dart' show Resolution;
-import '../compiler.dart' show Compiler;
 import '../constants/constructors.dart';
 import '../constants/expressions.dart';
 import '../core_types.dart' show CommonElements;
 import '../ordered_typeset.dart' show OrderedTypeSet;
 import '../resolution/scope.dart' show Scope;
 import '../resolution/tree_elements.dart' show TreeElements;
 import '../script.dart';
 import '../tokens/token.dart'
     show Token, isUserDefinableOperator, isMinusOperator;
 import '../tree/tree.dart';
@@ skipping 2024 matching lines @@
   /// by a field.
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }