Use UnmodifiableMapView in compiler

sdk/lib/_internal/compiler/implementation/dart2jslib.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 dart2js;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 
 import 'closure.dart' as closureMapping;
@@ skipping 21 matching lines @@
 import '../compiler.dart' as api;
 import 'patch_parser.dart';
 import 'types/types.dart' as ti;
 import 'resolution/resolution.dart';
 import 'resolution/class_members.dart' show MembersCreator;
 import 'source_file.dart' show SourceFile;
 import 'js/js.dart' as js;
 import 'deferred_load.dart' show DeferredLoadTask;
 import 'mirrors_used.dart' show MirrorUsageAnalyzerTask;
 import 'dump_info.dart';
-import 'helpers/helpers.dart';
 import 'tracer.dart' show Tracer;
 
 export 'resolution/resolution.dart' show TreeElements, TreeElementMapping;
 export 'scanner/scannerlib.dart' show isUserDefinableOperator,
                                       isUnaryOperator,
                                       isBinaryOperator,
                                       isTernaryOperator,
                                       isMinusOperator;
 export 'universe/universe.dart' show Selector, TypedSelector;
 export 'util/util.dart'
@@ skipping 10 matching lines @@
 part 'constant_system_dart.dart';
 part 'diagnostic_listener.dart';
 part 'enqueue.dart';
 part 'library_loader.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'tree_validator.dart';
 part 'typechecker.dart';
 part 'warnings.dart';
 part 'world.dart';