Create a Tree IR builder for JavaScript specific nodes.

pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart

 // Copyright (c) 2013, 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.
 
 // IrNodes are kept in a separate library to have precise control over their
 // dependencies on other parts of the system.
 library dart2js.ir_nodes;
 
 import '../constants/expressions.dart';
 import '../constants/values.dart' as values show ConstantValue;
@@ skipping 436 matching lines @@
   final Reference<Continuation> trueContinuation;
   final Reference<Continuation> falseContinuation;
 
   Branch(this.condition, Continuation trueCont, Continuation falseCont)
       : trueContinuation = new Reference<Continuation>(trueCont),
         falseContinuation = new Reference<Continuation>(falseCont);
 
   accept(Visitor visitor) => visitor.visitBranch(this);
 }
 
-class Identical extends Primitive {
+/// Marker interface for nodes that are only handled in the JavaScript backend.
+///
+/// These nodes are generated by the unsugar step and need special translation
+/// to the Tree IR, which is implemented in JsTreeBuilder.
+abstract class JsSpecificNode {}
+
+class Identical extends Primitive implements JsSpecificNode {
   final Reference<Primitive> left;
   final Reference<Primitive> right;
   Identical(Primitive left, Primitive right)
       : left = new Reference<Primitive>(left),
         right = new Reference<Primitive>(right);
   accept(Visitor visitor) => visitor.visitIdentical(this);
 }
 
-class Interceptor extends Primitive {
+class Interceptor extends Primitive implements JsSpecificNode {
   final Reference<Primitive> input;
   final Set<ClassElement> interceptedClasses;
   Interceptor(Primitive input, this.interceptedClasses)
       : this.input = new Reference<Primitive>(input);
   accept(Visitor visitor) => visitor.visitInterceptor(this);
 }
 
 class Constant extends Primitive {
   final ConstantExpression expression;
 
@@ skipping 607 matching lines @@
 
   void visitIdentical(Identical node) {
     visitReference(node.left);
     visitReference(node.right);
   }
 
   void visitInterceptor(Interceptor node) {
     visitReference(node.input);
   }
 }