Strong mode cleaning of many dart2js tests.

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2016, 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 mock_compiler;
 
 import 'dart:async';
 import 'dart:collection';
 
 import 'package:compiler/compiler_new.dart' as api;
 import 'package:compiler/src/common/names.dart' show Uris;
 import 'package:compiler/src/constants/expressions.dart';
 import 'package:compiler/src/elements/resolution_types.dart'
     show ResolutionDartType;
 import 'package:compiler/src/diagnostics/diagnostic_listener.dart';
 import 'package:compiler/src/diagnostics/source_span.dart';
 import 'package:compiler/src/diagnostics/spannable.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:compiler/src/elements/visitor.dart';
 import 'package:compiler/src/library_loader.dart' show LoadedLibraries;
-import 'package:compiler/src/js_backend/backend.dart' show JavaScriptBackend;
 import 'package:compiler/src/js_backend/lookup_map_analysis.dart'
     show LookupMapResolutionAnalysis;
 import 'package:compiler/src/io/source_file.dart';
 import 'package:compiler/src/options.dart' show CompilerOptions;
 import 'package:compiler/src/resolution/members.dart';
 import 'package:compiler/src/resolution/registry.dart';
 import 'package:compiler/src/resolution/scope.dart';
 import 'package:compiler/src/resolution/tree_elements.dart';
 import 'package:compiler/src/resolved_uri_translator.dart';
 import 'package:compiler/src/script.dart';
@@ skipping 362 matching lines @@
       trustTypeAnnotations: trustTypeAnnotations,
       enableTypeAssertions: enableTypeAssertions,
       enableUserAssertions: enableUserAssertions,
       expectedErrors: expectedErrors,
       expectedWarnings: expectedWarnings,
       outputProvider: outputProvider);
   compiler.registerSource(uri, code);
   compiler.diagnosticHandler = createHandler(compiler, code);
   return compiler;
 }