Put IR builder visitors in a different library than IrBuilder.

pkg/compiler/lib/src/dart2jslib.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;
 
 import 'dart:async';
 import 'dart:collection' show Queue;
 import 'dart:profiler' show
     UserTag;
 
 import '../compiler.dart' as api;
 import 'cache_strategy.dart';
 import 'closure.dart' as closureMapping;
 import 'constants/expressions.dart';
 import 'constants/values.dart';
 import 'core_types.dart';
-import 'cps_ir/cps_ir_builder.dart' show IrBuilderTask;
+import 'cps_ir/cps_ir_builder_task.dart' show IrBuilderTask;
 import 'dart_backend/dart_backend.dart' as dart_backend;
 import 'dart_types.dart';
 import 'deferred_load.dart' show DeferredLoadTask, OutputUnit;
 import 'dump_info.dart';
 import 'elements/elements.dart';
 import 'elements/modelx.dart'
     show ErroneousElementX,
          ClassElementX,
          CompilationUnitElementX,
          FunctionElementX,
@@ skipping 46 matching lines @@
 part 'compiler.dart';
 part 'constant_system.dart';
 part 'constant_system_dart.dart';
 part 'diagnostic_listener.dart';
 part 'enqueue.dart';
 part 'resolved_visitor.dart';
 part 'script.dart';
 part 'typechecker.dart';
 part 'warnings.dart';
 part 'world.dart';