Overhaul tree IR visitor and rename IR classes.

pkg/compiler/lib/src/js_backend/codegen/codegen.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 code_generator;
 
 import 'glue.dart';
 
 import '../../tree_ir/tree_ir_nodes.dart' as tree_ir;
 import '../../js/js.dart' as js;
 import '../../elements/elements.dart';
 import '../../io/source_information.dart' show SourceInformation;
 import '../../util/maplet.dart';
 import '../../constants/values.dart';
 import '../../dart2jslib.dart';
 
 class CodegenBailout {
   final tree_ir.Node node;
   final String reason;
   CodegenBailout(this.node, this.reason);
   String get message {
     return 'bailout${node != null ? " on $node" : ""}: $reason';
   }
 }
 
-class CodeGenerator extends tree_ir.Visitor<dynamic, js.Expression> {
+class CodeGenerator extends tree_ir.StatementVisitor
+                    with tree_ir.ExpressionVisitor<js.Expression> {
   final CodegenRegistry registry;
 
   final Glue glue;
 
   ExecutableElement currentFunction;
 
   /// Maps variables to their name.
   Map<tree_ir.Variable, String> variableNames = <tree_ir.Variable, String>{};
 
   /// Maps local constants to their name.
@@ skipping 551 matching lines @@
   @override
   visitFunctionExpression(tree_ir.FunctionExpression node) {
     return errorUnsupportedNode(node);
   }
 
   @override
   visitFunctionDeclaration(tree_ir.FunctionDeclaration node) {
     return errorUnsupportedNode(node);
   }
 
-  @override
-  visitFieldInitializer(tree_ir.FieldInitializer node) {
-    return errorUnsupportedNode(node);
-  }
-
-  @override
-  visitSuperInitializer(tree_ir.SuperInitializer node) {
-    return errorUnsupportedNode(node);
-  }
-
   errorUnsupportedNode(tree_ir.DartSpecificNode node) {
     throw "Unsupported node in JS backend: $node";
   }
 }