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

pkg/compiler/lib/src/compiler.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.compiler_base;
 
 import 'dart:async' show
     EventSink,
     Future;
 
 import '../compiler_new.dart' as api;
 import 'cache_strategy.dart' show
     CacheStrategy;
 import 'closure.dart' as closureMapping show
     ClosureTask;
+import 'common.dart';
 import 'common/backend_api.dart' show
     Backend;
 import 'common/codegen.dart' show
     CodegenRegistry,
     CodegenWorkItem;
 import 'common/names.dart' show
     Identifiers,
     Uris;
 import 'common/registry.dart' show
     Registry;
@@ skipping 13 matching lines @@
     CoreTypes;
 import 'dart_backend/dart_backend.dart' as dart_backend;
 import 'dart_types.dart' show
     DartType,
     DynamicType,
     InterfaceType,
     Types;
 import 'deferred_load.dart' show DeferredLoadTask, OutputUnit;
 import 'diagnostics/code_location.dart';
 import 'diagnostics/diagnostic_listener.dart' show
-    DiagnosticMessage,
-    DiagnosticOptions,
-    DiagnosticReporter;
+    DiagnosticOptions;
 import 'diagnostics/invariant.dart' show
-    invariant,
     REPORT_EXCESS_RESOLUTION;
 import 'diagnostics/messages.dart' show
     Message,
-    MessageKind,
     MessageTemplate;
-import 'diagnostics/source_span.dart' show
-    SourceSpan;
-import 'diagnostics/spannable.dart' show
-    CURRENT_ELEMENT_SPANNABLE,
-    NO_LOCATION_SPANNABLE,
-    Spannable,
-    SpannableAssertionFailure;
 import 'dump_info.dart' show
     DumpInfoTask;
 import 'elements/elements.dart';
 import 'elements/modelx.dart' show
     ErroneousElementX,
     ClassElementX,
     CompilationUnitElementX,
     DeferredLoaderGetterElementX,
     MethodElementX,
     LibraryElementX,
@@ skipping 1937 matching lines @@
 
   @override
   void parsePatchClass(ClassElement cls) {
     compiler.patchParser.measure(() {
       if (cls.isPatch) {
         compiler.patchParser.parsePatchClassNode(cls);
       }
     });
   }
 }