Fix dart2js warnings and add test to ensure it stays clean.

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 '../common_elements.dart';
 import '../common/backend_api.dart';
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/values.dart'
     show
         ConstantValue,
         ConstructedConstantValue,
         ListConstantValue,
         NullConstantValue,
         StringConstantValue,
         TypeConstantValue;
 import '../elements/elements.dart' show ClassElement, FieldElement;
 import '../elements/entities.dart';
 import '../elements/resolution_types.dart' show ResolutionInterfaceType;
-import '../options.dart';
 import '../universe/use.dart' show ConstantUse, StaticUse;
 import '../universe/world_impact.dart'
     show WorldImpact, StagedWorldImpactBuilder;
 import 'backend_helpers.dart';
 
 /// Lookup map handling for resolution.
 ///
 /// This analysis checks for the import of `package:lookup_map/lookup_map.dart`,
 /// and if found, read the `_version` variable from it.
 ///
@@ skipping 473 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');