Fix hints reported by analyzer.

pkg/compiler/lib/src/js_backend/lookup_map_analysis.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.
 
 /// Analysis to determine how to generate code for `LookupMap`s.
 library compiler.src.js_backend.lookup_map_analysis;
 
 import 'package:pub_semver/pub_semver.dart';
 
 import '../common.dart';
 import '../compiler.dart' show Compiler;
 import '../constants/values.dart'
     show
         ConstantValue,
         ConstructedConstantValue,
         ListConstantValue,
         NullConstantValue,
         StringConstantValue,
         TypeConstantValue;
-import '../elements/resolution_types.dart' show ResolutionDartType;
 import '../elements/resolution_types.dart' show ResolutionInterfaceType;
 import '../elements/elements.dart'
     show ClassElement, FieldElement, LibraryElement, VariableElement;
 import '../universe/use.dart' show StaticUse;
 import '../universe/world_impact.dart'
     show WorldImpact, StagedWorldImpactBuilder;
 import 'js_backend.dart' show JavaScriptBackend;
 
 /// An analysis and optimization to remove unused entries from a `LookupMap`.
 ///
@@ skipping 417 matching lines @@
         original.fields[analysis.valueField] = keyValuePairs[1];
       }
     } else {
       original.fields[analysis.entriesField] =
           new ListConstantValue(listType, keyValuePairs);
     }
   }
 }
 
 final _validLookupMapVersionConstraint = new VersionConstraint.parse('^0.0.1');