Refactor IrBuilder to use SemanticVisitor.

tests/compiler/dart2js/backend_dart/sexpr_test.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.
 
 library dart_backend.sexpr_test;
 
 import 'dart:async';
 
 import 'package:async_helper/async_helper.dart';
 import 'package:compiler/src/dart2jslib.dart';
@@ skipping 93 matching lines @@
 
 Future<List<String>> testStringifier(String code,
                                      Iterable<String> expectedTokens) {
   return compile(code)
       .then((List<FunctionDefinition> functions) {
         List<String> sexprs = stringifyAll(functions);
         String combined = sexprs.join();
         String withoutNullConstants = combined.replaceAll("Constant null", "");
         Expect.isFalse(withoutNullConstants.contains("null"));
         for (String token in expectedTokens) {
-          Expect.isTrue(combined.contains(token));
+          Expect.isTrue(combined.contains(token),
+              "'$combined' doesn't contain '$token' in test:\n$code");
         }
 
         return sexprs;
       });
 }
 
 /// Checks if the generated S-expressions can be processed by the unstringifier,
 /// returns the resulting definitions.
 List<FunctionDefinition> testUnstringifier(List<String> sexprs) {
   return sexprs.map((String sexpr) {
@@ skipping 39 matching lines @@
           ];
 
   asyncTest(() => testStringifier(CODE, tokens).then((List<String> sexprs) {
     final functions = testUnstringifier(sexprs);
 
     // Ensure that
     // stringified(CODE) == stringified(unstringified(stringified(CODE)))
     Expect.listEquals(sexprs, stringifyAll(functions));
   }));
 }