Support annotations on assignment and postfix operations

tests/compiler/dart2js/closure/data/mutations.dart

 // Copyright (c) 2017, 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.
 
 // Test that captured variables are boxed regardless of where they are mutated.
 
-/*mutateUnused:*/
+/*element: mutateUnused:*/
 mutateUnused() {
   var localVar;
   /**/ () {
     // Use nothing.
   };
   localVar = 42;
   return localVar;
 }
 
-/*mutateInClosure:box=(box0 which holds [localVar])*/
+/*element: mutateInClosure:box=(box0 which holds [localVar])*/
 mutateInClosure() {
   var /*boxed*/ localVar;
   /*free=[box0,localVar]*/ () {
     localVar = 43;
   };
   return localVar;
 }
 
-/*mutateOutsideClosure:box=(box0 which holds [localVar])*/
+/*element: mutateOutsideClosure:box=(box0 which holds [localVar])*/
 mutateOutsideClosure() {
   var /*boxed*/ localVar;
   /*free=[box0,localVar]*/ () {
     print(localVar);
   };
   localVar = 44;
   return localVar;
 }
 
-/*mutateInOtherClosure:box=(box0 which holds [localVar])*/
+/*element: mutateInOtherClosure:box=(box0 which holds [localVar])*/
 mutateInOtherClosure() {
   var /*boxed*/ localVar;
   /*free=[box0,localVar]*/ () {
     print(localVar);
   };
   /*free=[box0,localVar]*/ () {
     localVar = 45;
   };
   return localVar;
 }
 
-/*mutateInNestedClosure:box=(box0 which holds [localVar])*/
+/*element: mutateInNestedClosure:box=(box0 which holds [localVar])*/
 mutateInNestedClosure() {
   var /*boxed*/ localVar;
   /*free=[box0,localVar]*/ () {
     print(localVar);
     /*free=[box0,localVar]*/ () {
       localVar = 46;
     };
   };
   return localVar;
 }
 
 main() {
   mutateUnused();
   mutateInClosure();
   mutateOutsideClosure();
   mutateInOtherClosure();
   mutateInNestedClosure();
 }