Add CPS IR transformation to make the JavaScript backend specific semantics explicit in the tree.

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

Index: pkg/compiler/lib/src/cps_ir/constant_propagation.dart
diff --git a/pkg/compiler/lib/src/cps_ir/constant_propagation.dart b/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
index fc9ebba0fed2f353a69aade7efe5ce42d1bc860f..daadc5be55bc3d349502d3a5fd014ee564c18977 100644
--- a/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
+++ b/pkg/compiler/lib/src/cps_ir/constant_propagation.dart
@@ -28,7 +28,7 @@ class ConstantPropagator implements Pass {
 
     // Set all parent pointers.
 
-    new _ParentVisitor().visit(root);
+    new ParentVisitor().visit(root);
 
     // Analyze. In this phase, the entire term is analyzed for reachability
     // and the constant status of each expression.
@@ -603,6 +603,12 @@ class _ConstPropagationVisitor extends Visitor {
     Branch branch = node.parent;
     visitBranch(branch);
   }
+
+  // JavaScript specific nodes.
+
+  void visitIdentical(Identical node) {
+    setValue(node, _ConstnessLattice.NonConst);
+  }
 }
 
 /// Represents the constant-state of a variable at some point in the program.