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

pkg/compiler/lib/src/enqueue.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.enqueue;
 
 import 'dart:collection' show
     Queue;
 
+import 'common.dart';
 import 'common/names.dart' show
     Identifiers;
 import 'common/resolution.dart' show
     Resolution;
 import 'common/work.dart' show
     ItemCompilationContext,
     WorkItem;
 import 'common/tasks.dart' show
     CompilerTask,
     DeferredAction,
     DeferredTask;
 import 'common/codegen.dart' show
     CodegenWorkItem;
 import 'common/resolution.dart' show
     ResolutionWorkItem;
 import 'compiler.dart' show
     Compiler;
 import 'dart_types.dart' show
     DartType,
     InterfaceType;
-import 'diagnostics/diagnostic_listener.dart' show
-    DiagnosticReporter;
-import 'diagnostics/invariant.dart' show
-    invariant;
-import 'diagnostics/spannable.dart' show
-    SpannableAssertionFailure;
 import 'elements/elements.dart' show
     AnalyzableElement,
     AstElement,
     ClassElement,
     ConstructorElement,
     Element,
     Elements,
     FunctionElement,
     LibraryElement,
     LocalFunctionElement,
     Member,
     MemberElement,
     MethodElement,
     Name,
     TypedElement,
     TypedefElement;
 import 'js/js.dart' as js;
 import 'native/native.dart' as native;
-import 'resolution/members.dart' show
-    ResolverVisitor;
-import 'tree/tree.dart' show
-    Send;
 import 'types/types.dart' show
     TypeMaskStrategy;
 import 'universe/selector.dart' show
     Selector;
 import 'universe/universe.dart';
 import 'util/util.dart' show
     Link,
     Setlet;
 
 typedef ItemCompilationContext ItemCompilationContextCreator();
@@ skipping 1024 matching lines @@
   @override
   void processStaticUse(Enqueuer enqueuer, Element element) {
     enqueuer.registerStaticUseInternal(element);
   }
 
   @override
   void processSelector(Enqueuer enqueuer, UniverseSelector selector) {
     enqueuer.handleUnseenSelectorInternal(selector);
   }
 }