MutableVariable no longer has a host Element.

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 694 matching lines @@
   /// (CreateFunction (definition))
   CreateFunction parseCreateFunction() {
     tokens.consumeStart(CREATE_FUNCTION);
     FunctionDefinition def = parseFunctionDefinition();
     tokens.consumeEnd();
     return new CreateFunction(def);
   }
 
   MutableVariable addMutableVariable(String name) {
     assert(!name2variable.containsKey(name));
-    MutableVariable variable =
-        new MutableVariable(new DummyElement(""), new DummyElement(name));
+    MutableVariable variable = new MutableVariable(new DummyElement(name));
     name2variable[name] = variable;
     return variable;
   }
 
   /// (GetMutableVariable name)
   GetMutableVariable parseGetMutableVariable() {
     tokens.consumeStart(GET_MUTABLE_VARIABLE);
     MutableVariable local = name2variable[tokens.read()];
     tokens.consumeEnd();
 
@@ skipping 37 matching lines @@
   /// (GetStatic field)
   GetStatic parseGetStatic() {
     tokens.consumeStart(GET_STATIC);
 
     Element field = new DummyElement(tokens.read());
 
     tokens.consumeEnd();
     return new GetStatic(field, null);
   }
 }