Create a Tree IR builder for JavaScript specific nodes.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
index f6ca56bfc34076969630ddc08aaab9893b253706..ffc975a87b12dc758986537ea24de4ac2c6e1e04 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_nodes.dart
@@ -454,7 +454,13 @@ class Branch extends Expression {
   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)
@@ -463,7 +469,7 @@ class Identical extends Primitive {
   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)