Support annotations on assignment and postfix operations

tests/compiler/dart2js/closure/data/nested_closures.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 boxing/captures for nested closures.
 
-/*useOne:box=(box0 which holds [b1])*/ useOne(/*boxed*/ b1) {
+/*element: useOne:box=(box0 which holds [b1])*/ useOne(/*boxed*/ b1) {
   /*box=(box1 which holds [b2]),free=[b1,box0]*/ () {
     var /*boxed*/ b2 = (b1 = 1);
 
     /*free=[b2,box1]*/ () {
       return (b2 = 2);
     };
 
     return b2;
   };
   return b1;
 }
 
-/*useBoth:box=(box0 which holds [b1])*/ useBoth(/*boxed*/ b1) {
+/*element: useBoth:box=(box0 which holds [b1])*/ useBoth(/*boxed*/ b1) {
   /*box=(box1 which holds [b2]),free=[b1,box0]*/ () {
     var /*boxed*/ b2 = (b1 = 1);
 
     /*free=[b1,b2,box0,box1]*/ () {
       return b1 + (b2 = 2);
     };
 
     return b2;
   };
   return b1;
 }
 
-/*useMany:box=(box0 which holds [b1,b2,b3])*/ useMany(c1, /*boxed*/ b1) {
+/*element: useMany:box=(box0 which holds [b1,b2,b3])*/
+useMany(c1, /*boxed*/ b1) {
   var /*boxed*/ b2 = 2;
   var /*boxed*/ b3 = 3;
   var c2 = 2;
   var c3 = 3;
   /*box=(box1 which holds [b4]),free=[b1,b2,b3,box0,c1,c2,c3]*/ () {
     var c4 = c1 + c2 + c3;
     var /*boxed*/ b4 = (b1 = 1) + (b2 = 2) + (b3 = 3);
 
     /*free=[b1,b2,b4,box0,box1,c4]*/ () {
       return c4 + (b1 = 1) + (b2 = 2) + (b4 = 4);
     };
 
     return b4;
   };
   return b1 + b2 + b3 + c1 + c2 + c3;
 }
 
 main() {
   useOne(1);
   useBoth(1);
   useMany(1, 2);
 }