dart2js cps: Translate identity placeholders to refinement nodes.

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

Index: pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
diff --git a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
index da57c4a7a32ad56c0d5fe16ba445588e199a2e2a..c90cfa092c59ff8682a08c6d0ff47a616af882e1 100644
--- a/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
+++ b/pkg/compiler/lib/src/cps_ir/cps_ir_builder.dart
@@ -21,7 +21,8 @@ import '../io/source_information.dart';
 import '../js/js.dart' as js show
     js,
     LiteralStatement,
-    Template;
+    Template,
+    isIdentityTemplate;
 import '../native/native.dart' show
     NativeBehavior;
 import '../tree/tree.dart' as ast;
@@ -2702,6 +2703,13 @@ class IrBuilder {
                                 {Element dependency}) {
     assert(behavior != null);
     TypeMask type = program.getTypeMaskForForeign(behavior);
+    if (js.isIdentityTemplate(codeTemplate) && !program.isArrayType(type)) {
+      // JS expression is just a refinement.
+      // Do not do this for arrays - those are special because array types can
+      // change at runtime. The input and output must therefore be modeled
+      // as distinct values.
+      return addPrimitive(new ir.Refinement(arguments.single, type));
+    }
     ir.Primitive result = addPrimitive(new ir.ForeignCode(
         codeTemplate,
         type,