Overhaul tree IR visitor and rename IR classes.

tests/compiler/dart2js/backend_dart/sexpr2_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.
 
 /// Unittest test of the CPS ir generated by the dart2dart compiler.
 library dart_backend.sexpr2_test;
 
 import 'package:compiler/src/dart2jslib.dart';
 import 'package:compiler/src/cps_ir/cps_ir_nodes.dart';
 import 'package:compiler/src/cps_ir/cps_ir_nodes_sexpr.dart';
 import 'package:compiler/src/elements/elements.dart';
 import 'package:expect/expect.dart';
 
 import '../../../../pkg/analyzer2dart/test/test_helper.dart';
 import '../../../../pkg/analyzer2dart/test/sexpr_data.dart';
 
 import 'test_helper.dart';
 
 main(List<String> args) {
   performTests(TEST_DATA, asyncTester, runTest, args);
 }
 
 runTest(TestSpec result) {
   return compilerFor(result.input).then((Compiler compiler) {
     void checkOutput(String elementName,
                      Element element,
                      String expectedOutput) {
-      ExecutableDefinition ir = compiler.irBuilder.getIr(element);
+      RootNode ir = compiler.irBuilder.getIr(element);
       if (expectedOutput == null) {
         Expect.isNull(ir, "\nInput:\n${result.input}\n"
                           "No CPS IR expected for $element");
       } else {
         Expect.isNotNull(ir, "\nInput:\n${result.input}\n"
                              "No CPS IR for $element");
         expectedOutput = expectedOutput.trim();
         String output = ir.accept(new SExpressionStringifier()).trim();
         Expect.equals(expectedOutput, output,
             "\nInput:\n${result.input}\n"
@@ skipping 14 matching lines @@
           element = cls.localLookup(
               elementName.substring(elementName.indexOf('.') + 1));
         } else {
           element = compiler.mainApp.localLookup(elementName);
         }
         checkOutput(elementName, element, output);
       });
     }
   });
 }