dart2js cps: Handle error cases.

tests/compiler/dart2js/analyze_unused_dart2js_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.
 
 library analyze_unused_dart2js;
 
 import 'package:async_helper/async_helper.dart';
 
 import 'package:compiler/src/dart2jslib.dart';
 import 'package:compiler/src/filenames.dart';
@@ skipping 11 matching lines @@
       "The method 'asLiteralBool' is never called"],
 
   // Some things in dart_printer are not yet used
   "lib/src/dart_backend/backend_ast_nodes.dart": const [" is never "],
 
   // Uncalled error methods in SemanticSendVisitor and subclasses.
   "lib/src/resolution/semantic_visitor.dart": const [
       "The method 'error"],
   "lib/src/resolution/semantic_visitor_mixins.dart": const [
       "The method 'error"],
+  "lib/src/cps_ir/cps_ir_builder_task.dart": const [
+      "The method 'error"],
 
   // Uncalled type predicate.  Keep while related predicates are used.
   "lib/src/ssa/nodes.dart": const [
       "The method 'isArray' is never called"],
 };
 
 void main() {
   var uri = currentDirectory.resolve(
       'pkg/compiler/lib/src/use_unused_api.dart');
   asyncTest(() => analyze([uri], WHITE_LIST,
@@ skipping 19 matching lines @@
     } else if (member.isTypedef) {
       if (member.isResolved) {
         compiler.reportHint(member, MessageKind.GENERIC,
             {'text': "Helper typedef in production code '$member'."});
       }
     }
   }
   compiler.libraryLoader.lookupLibrary(helperUri).forEachLocalMember(checkLive);
   return handler.checkResults();
 }