Refactor the CPS SExprStringifier.

tests/compiler/dart2js/backend_dart/sexpr_unstringifier.dart

 // Copyright (c) 2014, 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.
 
 // SExpressionUnstringifier implements the inverse operation to
 // [SExpressionStringifier].
 
 library sexpr_unstringifier;
 
 import 'package:compiler/src/constants/expressions.dart'
@@ skipping 21 matching lines @@
 }
 
 /// Used whenever a node constructed by [SExpressionUnstringifier] needs a
 /// local.
 class DummyLocal extends DummyEntity implements Local {
   DummyLocal(String name) : super(name);
 
   ExecutableElement get executableContext => null;
 }
 
+// TODO(karlklose): we should remove all references to [ErroneousElement] from
+// the CPS IR.  Instead, the builder must construct appropriate terms for ASTs
+// that could not be resolved correctly.  Perhaps the IR should not rely on
+// elements at all for naming.
 /// Used whenever a node constructed by [SExpressionUnstringifier] requires
 /// an [Element] or [FunctionElement]. Extends [ErroneousElementX] since there
 /// is currently a large amount of overhead when extending the base abstract
 /// classes, and erroneous elements conveniently also skip several assertion
 /// checks in CPS IR nodes that are irrelevant to us.
 class DummyElement extends ErroneousElementX
     implements TypeVariableElement {
   DummyElement(String name)
       : super(dart2js.MessageKind.GENERIC, {}, name, null);
 
@@ skipping 626 matching lines @@
     return new ReifyTypeVar(type);
   }
 
   /// (This)
   This parseThis() {
     tokens.consumeStart(THIS);
     tokens.consumeEnd();
     return new This();
   }
 }