Overhaul tree IR visitor and rename IR classes.

pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart

Index: pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart
diff --git a/pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart b/pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart
index ba21ccbd942adbb99ef0f13c6d1d2e65ef2675b5..bc4867dd3e83eb34393f19a117fc7b9d2b59e892 100644
--- a/pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart
+++ b/pkg/compiler/lib/src/dart_backend/backend_ast_nodes.dart
@@ -26,11 +26,11 @@ 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);
@@ -322,7 +322,7 @@ class SuperInitializer extends Initializer {
   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;