Cleanup 1: split parts into their own libraries, remove unused imports

pkg/compiler/lib/src/universe/world_impact.dart

 // Copyright (c) 2015, 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.universe.world_impact;
 
-import '../dart_types.dart' show
-    DartType,
-    InterfaceType;
 import '../elements/elements.dart' show
     Element,
     LocalFunctionElement,
     MethodElement;
 import '../util/util.dart' show
     Setlet;
 
 import 'use.dart' show
     DynamicUse,
     StaticUse,
@@ skipping 221 matching lines @@
   }
 
   @override
   void visitTypeUse(TypeUse use) {
     if (_visitTypeUse != null) {
       _visitTypeUse(use);
     }
   }
 }