Add more tests for boxed variables.

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:box=box0,boxed=[b1,b2,b3]*/ test(c1, /*boxed*/ b1) {
+/// Test boxing/captures for nested closures.
+
+/*useOne:box=box0,boxed=[b1]*/ useOne(/*boxed*/ b1) {
+  /*box=box1,boxed=[b2],free=[b1,box0]*/ () {
+    var /*boxed*/ b2 = (b1 = 1);
+
+    /*free=[b2,box1]*/ () {
+      return (b2 = 2);
+    };
+
+    return b2;
+  };
+  return b1;
+}
+
+/*useBoth:box=box0,boxed=[b1]*/ useBoth(/*boxed*/ b1) {
+  /*box=box1,boxed=[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,boxed=[b1,b2,b3]*/ useMany(c1, /*boxed*/ b1) {
   var /*boxed*/ b2 = 2;
   var /*boxed*/ b3 = 3;
   var c2 = 2;
   var c3 = 3;
   /*box=box1,boxed=[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() {
-  test(1, 2);
+  useOne(1);
+  useBoth(1);
+  useMany(1, 2);
 }