Overhaul tree IR visitor and rename IR classes.

tests/compiler/dart2js/js_backend_cps_ir_source_information_test.dart

 // Copyright (c) 2015, 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.
 // VMOptions=-DUSE_CPS_IR=true
 
 // Test that the CPS IR code generator generates source information.
 
 library source_information_tests;
 
 import 'package:async_helper/async_helper.dart';
@@ skipping 22 matching lines @@
 
 String formatTest(Map test) {
   return test[TEST_MAIN_FILE];
 }
 
 js.Node getCodeForMain(Compiler compiler) {
   Element mainFunction = compiler.mainFunction;
   return compiler.enqueuer.codegen.generatedCode[mainFunction];
 }
 
-js.Node getJsNodeForElement(Compiler compiler,
-                                Element element) {
+js.Node getJsNodeForElement(Compiler compiler, Element element) {
   return compiler.enqueuer.codegen.generatedCode[element];
 }
 
-ir.ExecutableDefinition getIrNodeForElement(Compiler compiler,
-                                            Element element) {
+ir.RootNode getIrNodeForElement(Compiler compiler, Element element) {
   return compiler.irBuilder.getIr(element);
 }
 
-String getCodeForMethod(Compiler compiler,
-                        String name) {
+String getCodeForMethod(Compiler compiler, String name) {
   Element foundElement;
   for (Element element in compiler.enqueuer.codegen.generatedCode.keys) {
     if (element.toString() == name) {
       if (foundElement != null) {
         Expect.fail('Multiple compiled elements are called $name');
       }
       foundElement = element;
     }
   }
 
@@ skipping 75 matching lines @@
   print('Hello');
   print('World');
 }
 """, const ['memory:test.dart:[2,3]',
             'memory:test.dart:[3,3]']),
 ];
 
 void main() {
   runTests(tests);
 }