Add boxed fields in closure classes.

tests/compiler/dart2js/kernel/compile_from_dill_test_helper.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.
 
 // Helper to test compilation equivalence between source and .dill based
 // compilation.
 library dart2js.kernel.compile_from_dill_test_helper;
 
 import 'dart:async';
 
@@ skipping 42 matching lines @@
 
 foo({named}) => 1;
 bar(a) => !a;
 class Class {
   var field;
   static var staticField;
 
   Class();
   Class.named(this.field) {
     staticField = 42;
-  } 
+  }
 
   method() {}
 }
- 
+
 class SubClass extends Class {
   method() {
     super.method();
-  }  
+  }
 }
 
 class Generic<T> {
   method(o) => o is T;
 }
 
 var toplevel;
 
 main() {
   foo();
@@ skipping 56 matching lines @@
 typedef NoArg();
 @NoInline()
 testIs(o) => o is Generic<int> || o is NoArg;
 @NoInline()
 testAsGeneric(o) => o as Generic<int>;
 @NoInline()
 testAsFunction(o) => o as NoArg;
 '''
   }),
   const Test(const {
-    'main.dart': ''' 
+    'main.dart': '''
 
 main() {
   var x;
   int i = 0;
   do {
     if (i == 5) continue;
     x = i;
     if (i == 7) break;
     i++;
   } while (i < 10);
   print(x);
-} 
+}
 '''
   }, expectIdenticalOutput: false),
   const Test(const {
     'main.dart': '''
 class A<U,V> {
   var a = U;
   var b = V;
 }
 class B<Q, R> extends A<R, W<Q>> {
 }
@@ skipping 188 matching lines @@
       Map<String, BufferedOutputSink> map2 = collector2.outputMap[outputType];
       checkSets(map1.keys, map2.keys, 'output', equality);
       map1.forEach((String name, BufferedOutputSink output1) {
         BufferedOutputSink output2 = map2[name];
         Expect.stringEquals(output1.text, output2.text);
       });
     });
   }
   return ResultKind.success;
 }