Put IR builder visitors in a different library than IrBuilder.

pkg/compiler/lib/src/js_backend/codegen/task.dart

 // Copyright (c) 2014, 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.
 
 /// Generate code using the cps-based IR pipeline.
 library code_generator_task;
 
 import 'glue.dart';
 import 'codegen.dart';
 import 'unsugar.dart';
 
 import '../js_backend.dart';
 import '../../dart2jslib.dart';
 import '../../cps_ir/cps_ir_nodes.dart' as cps;
-import '../../cps_ir/cps_ir_builder.dart';
+import '../../cps_ir/cps_ir_builder_task.dart';
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../types/types.dart' show TypeMask, UnionTypeMask, FlatTypeMask,
     ForwardingTypeMask;
 import '../../elements/elements.dart';
 import '../../js/js.dart' as js;
 import '../../io/source_information.dart' show StartEndSourceInformation;
 import '../../tree_ir/tree_ir_builder.dart' as tree_builder;
 import '../../dart_backend/backend_ast_emitter.dart' as backend_ast_emitter;
 import '../../cps_ir/optimizers.dart';
 import '../../tracer.dart';
@@ skipping 177 matching lines @@
   Iterable<CompilerTask> get tasks {
     // TODO(sigurdm): Make a better list of tasks.
     return <CompilerTask>[irBuilderTask]..addAll(fallbackCompiler.tasks);
   }
 
   js.Node attachPosition(js.Node node, AstElement element) {
     return node.withSourceInformation(
         StartEndSourceInformation.computeSourceInformation(element));
   }
 }