Use common.dart to re-export commonly used entities.

pkg/compiler/lib/src/js_backend/js_backend.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 js_backend;
 
 import 'dart:async' show EventSink, Future;
 import 'dart:collection' show HashMap;
 
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 import 'package:js_runtime/shared/embedded_names.dart' show JsGetName;
 
 import '../closure.dart';
+import '../common.dart';
 import '../common/backend_api.dart' show
     Backend,
     ForeignResolver;
 import '../common/codegen.dart' show
     CodegenRegistry,
     CodegenWorkItem;
 import '../common/names.dart' show
     Identifiers,
     Selectors,
     Uris;
@@ skipping 11 matching lines @@
     TransformedWorldImpact;
 import '../common/work.dart' show
     ItemCompilationContext;
 import '../compiler.dart' show
     Compiler;
 import '../compile_time_constants.dart';
 import '../constants/constant_system.dart';
 import '../constants/expressions.dart';
 import '../constants/values.dart';
 import '../dart_types.dart';
-import '../diagnostics/diagnostic_listener.dart' show
-    DiagnosticMessage,
-    DiagnosticReporter;
-import '../diagnostics/invariant.dart' show
-    invariant;
-import '../diagnostics/messages.dart' show MessageKind;
-import '../diagnostics/spannable.dart' show
-    CURRENT_ELEMENT_SPANNABLE,
-    NO_LOCATION_SPANNABLE,
-    Spannable,
-    SpannableAssertionFailure;
 import '../elements/elements.dart';
 import '../elements/visitor.dart' show
     BaseElementVisitor;
 import '../enqueue.dart' show
     Enqueuer,
     ResolutionEnqueuer,
     WorldImpact;
 import '../io/code_output.dart';
 import '../io/source_information.dart' show
     SourceInformationStrategy,
@@ skipping 44 matching lines @@
 part 'constant_handler_javascript.dart';
 part 'custom_elements_analysis.dart';
 part 'frequency_namer.dart';
 part 'field_naming_mixin.dart';
 part 'minify_namer.dart';
 part 'namer.dart';
 part 'namer_names.dart';
 part 'no_such_method_registry.dart';
 part 'runtime_types.dart';
 part 'type_variable_handler.dart';