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

pkg/compiler/lib/src/common/backend_api.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.backend_api;
 
 import 'dart:async' show Future;
 
+import '../common.dart';
 import '../compiler.dart' show
     Compiler;
 import '../compile_time_constants.dart' show
     BackendConstantEnvironment,
     ConstantCompilerTask;
 import '../constants/expressions.dart' show
     ConstantExpression;
 import '../constants/constant_system.dart' show
     ConstantSystem;
 import '../constants/values.dart' show
     ConstantValue;
 import '../dart_types.dart' show
     DartType,
     InterfaceType;
-import '../diagnostics/spannable.dart' show
-    Spannable,
-    SpannableAssertionFailure;
 import '../elements/elements.dart' show
     ClassElement,
     ConstructorElement,
     Element,
     FunctionElement,
     LibraryElement,
     MetadataAnnotation;
 import '../enqueue.dart' show
     Enqueuer,
     CodegenEnqueuer,
@@ skipping 359 matching lines @@
   /// Returns the constant expression of [node], or `null` if [node] is not
   /// a constant expression.
   ConstantExpression getConstant(Node node);
 
   /// Registers [type] as instantiated.
   void registerInstantiatedType(InterfaceType type);
 
   /// Resolves [typeName] to a type in the context of [node].
   DartType resolveTypeFromString(Node node, String typeName);
 }