Put IR builder visitors in a different library than IrBuilder.

pkg/compiler/lib/src/use_unused_api.dart

Index: pkg/compiler/lib/src/use_unused_api.dart
diff --git a/pkg/compiler/lib/src/use_unused_api.dart b/pkg/compiler/lib/src/use_unused_api.dart
index 909b0003dba690b2ddfe6b03f2140bd8919c23f0..adc7a7e06f0091f3507d5d86a67d1bfa83608605 100644
--- a/pkg/compiler/lib/src/use_unused_api.dart
+++ b/pkg/compiler/lib/src/use_unused_api.dart
@@ -13,6 +13,7 @@ import '../compiler.dart' as api;
 import 'colors.dart' as colors;
 import 'constants/values.dart' as constants;
 import 'cps_ir/cps_ir_builder.dart' as ir_builder;
+import 'cps_ir/cps_ir_builder_task.dart' as ir_builder;
 import 'cps_ir/cps_ir_nodes_sexpr.dart' as cps_ir_nodes_sexpr;
 import 'dart_types.dart' as dart_types;
 import 'dart2js.dart' as dart2js;