Further reduce use of Compiler.closedWorld.

tests/compiler/dart2js/simple_inferrer_callers_test.dart

 // Copyright (c) 2013, 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.
 
 // Test that computation of callers of an element works when two
 // elements of the same name are being invoked in the same method.
 
 import 'package:async_helper/async_helper.dart';
 import 'package:expect/expect.dart';
 import 'package:compiler/src/inferrer/type_graph_inferrer.dart';
-import 'package:compiler/src/world.dart' show ClosedWorldRefiner;
+import 'package:compiler/src/world.dart' show ClosedWorld, ClosedWorldRefiner;
 
 import 'compiler_helper.dart';
 
 const String TEST = """
 class A {
   var field;
 }
 
 class B {
   var field;
@@ skipping 11 matching lines @@
   MyInferrer(compiler, closedWorld, closedWorldRefiner)
       : super(compiler, closedWorld, closedWorldRefiner);
   clear() {}
 }
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri, analyzeOnly: true);
   asyncTest(() => compiler.run(uri).then((_) {
         ClosedWorldRefiner closedWorldRefiner = compiler.closeResolution();
+        ClosedWorld closedWorld = compiler.closedWorld;
         var inferrer =
-            new MyInferrer(compiler, compiler.closedWorld, closedWorldRefiner);
+            new MyInferrer(compiler, closedWorld, closedWorldRefiner);
         compiler.globalInference.typesInferrerInternal = inferrer;
         compiler.globalInference.runGlobalTypeInference(
-            compiler.mainFunction, compiler.closedWorld, closedWorldRefiner);
+            compiler.mainFunction, closedWorld, closedWorldRefiner);
         var mainElement = findElement(compiler, 'main');
         var classA = findElement(compiler, 'A');
         var fieldA = classA.lookupLocalMember('field');
         var classB = findElement(compiler, 'B');
         var fieldB = classB.lookupLocalMember('field');
 
         Expect.isTrue(inferrer.getCallersOf(fieldA).contains(mainElement));
         Expect.isTrue(inferrer.getCallersOf(fieldB).contains(mainElement));
       }));
 }