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

pkg/compiler/lib/src/apiimpl.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 leg_apiimpl;
 
 import 'dart:async';
 import 'dart:convert';
 
 import 'package:package_config/packages.dart';
 import 'package:package_config/packages_file.dart' as pkgs;
 import 'package:package_config/src/packages_impl.dart' show
     MapPackages,
     NonFilePackagesDirectoryPackages;
 import 'package:package_config/src/util.dart' show
     checkValidPackageUri;
 import 'package:sdk_library_metadata/libraries.dart' hide LIBRARIES;
 import 'package:sdk_library_metadata/libraries.dart' as library_info show
     LIBRARIES;
 
 import '../compiler_new.dart' as api;
 import 'commandline_options.dart';
+import 'common.dart';
 import 'common/tasks.dart' show
     GenericTask;
 import 'compiler.dart' as leg;
 import 'diagnostics/diagnostic_listener.dart' show
-    DiagnosticMessage,
     DiagnosticOptions;
-import 'diagnostics/messages.dart';
-import 'diagnostics/source_span.dart' show
-    SourceSpan;
-import 'diagnostics/spannable.dart' show
-    NO_LOCATION_SPANNABLE,
-    Spannable;
+import 'diagnostics/messages.dart' show
+    Message;
 import 'elements/elements.dart' as elements;
 import 'io/source_file.dart';
 import 'script.dart';
 
 const bool forceIncrementalSupport =
     const bool.fromEnvironment('DART2JS_EXPERIMENTAL_INCREMENTAL_SUPPORT');
 
 class Compiler extends leg.Compiler {
   api.CompilerInput provider;
   api.CompilerDiagnostics handler;
@@ skipping 499 matching lines @@
     }
   }
 
 
   fromEnvironment(String name) => environment[name];
 
   LibraryInfo lookupLibraryInfo(String libraryName) {
     return library_info.LIBRARIES[libraryName];
   }
 }