dart2js cps: Translate identity placeholders to refinement nodes.

pkg/compiler/lib/src/cps_ir/cps_ir_builder.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.
 
 library dart2js.ir_builder;
 
 import '../closure.dart' as closure;
 import '../common.dart';
 import '../common/names.dart' show
     Names,
     Selectors;
 import '../compile_time_constants.dart' show
     BackendConstantEnvironment;
 import '../constants/constant_system.dart';
 import '../constants/values.dart' show
     ConstantValue,
     PrimitiveConstantValue;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 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;
 import '../types/types.dart' show
     TypeMask;
 import '../universe/call_structure.dart' show
     CallStructure;
 import '../universe/selector.dart' show
     Selector,
     SelectorKind;
@@ skipping 2660 matching lines @@
                                      List<ir.Primitive> arguments) {
     return addPrimitive(new ir.CreateInvocationMirror(selector, arguments));
   }
 
   ir.Primitive buildForeignCode(js.Template codeTemplate,
                                 List<ir.Primitive> arguments,
                                 NativeBehavior behavior,
                                 {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,
         arguments,
         behavior,
         dependency: dependency));
     if (!codeTemplate.isExpression) {
       // Close the term if this is a "throw" expression or native body.
       add(new ir.Unreachable());
       _current = null;
@@ skipping 184 matching lines @@
 }
 
 class SwitchCaseInfo {
   final List<ir.Primitive> constants = <ir.Primitive>[];
   final SubbuildFunction buildBody;
 
   SwitchCaseInfo(this.buildBody);
 
   void addConstant(ir.Primitive constant) => constants.add(constant);
 }