Overhaul tree IR visitor and rename IR classes.

pkg/compiler/lib/src/dart_backend/backend_ast_nodes.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 backend_ast_nodes;
 
 import '../constants/values.dart' as values;
 import '../dart_types.dart' as types;
 import '../elements/elements.dart' as elements;
 import '../tree/tree.dart' as tree;
 import '../util/characters.dart' as characters;
 
 /// The following nodes correspond to [tree.Send] expressions:
 /// [FieldExpression], [IndexExpression], [Assignment], [Increment],
 /// [CallFunction], [CallMethod], [CallNew], [CallStatic], [UnaryOperator],
 /// [BinaryOperator], and [TypeOperator].
 abstract class Node {}
 
 /// Receiver is an [Expression] or the [SuperReceiver].
 abstract class Receiver extends Node {}
 
 /// Argument is an [Expression] or a [NamedArgument].
 abstract class Argument extends Node {}
 
 abstract class Expression extends Node implements Receiver, Argument {
   bool get assignable => false;
 }
 
-abstract class ExecutableDefinition extends Node {
+abstract class RootNode extends Node {
   elements.Element get element;
 }
 
-class FieldDefinition extends ExecutableDefinition {
+class FieldDefinition extends RootNode {
   final elements.Element element;
   final Expression initializer;
   FieldDefinition(this.element, this.initializer);
 }
 
 abstract class Statement extends Node {}
 
 /// Used as receiver in expressions that dispatch to the super class.
 /// For instance, an expression such as `super.f()` is represented
 /// by a [CallMethod] node with [SuperReceiver] as its receiver.
@@ skipping 271 matching lines @@
   FieldInitializer(this.element, this.body);
 }
 
 class SuperInitializer extends Initializer {
   final elements.ConstructorElement target;
   final List<Argument> arguments;
 
   SuperInitializer(this.target, this.arguments);
 }
 
-class FunctionExpression extends Expression implements ExecutableDefinition {
+class FunctionExpression extends Expression implements RootNode {
   final TypeAnnotation returnType;
   String name;
   final Parameters parameters;
   final Statement body;
   final bool isGetter;
   final bool isSetter;
 
   elements.FunctionElement element;
 
   FunctionExpression(this.parameters,
@@ skipping 1234 matching lines @@
   final StringChunk previous;
   final tree.StringQuoting quoting;
   num cost;
 
   OpenStringChunk(this.previous, this.quoting, this.cost);
 
   StringChunk end(int endIndex) {
     return new StringChunk(previous, quoting, endIndex);
   }
 }