Implement proper scoping for "this" in arrow functions

test/mjsunit/debug-scopes.js

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 149 matching lines @@
     }
     count++;
   }
 
   // 'arguments' and might be exposed in the local and closure scope. Just
   // ignore this.
   var scope_size = scope.scopeObject().properties().length;
   if (!scope.scopeObject().property('arguments').isUndefined()) {
     scope_size--;
   }
+  // Ditto for 'this'.
+  if (!scope.scopeObject().property('this').isUndefined()) {
+    scope_size--;
+  }
   // Skip property with empty name.
   if (!scope.scopeObject().property('').isUndefined()) {
     scope_size--;
   }
 
   if (count != scope_size) {
     print('Names found in scope:');
     var names = scope.scopeObject().propertyNames();
     for (var i = 0; i < names.length; i++) {
       print(names[i]);
@@ skipping 866 matching lines @@
   CheckScopeContent({e:'Exception'}, 0, exec_state);
 };
 catch_block_7();
 EndTest();
 
 
 assertEquals(begin_test_count, break_count,
              'one or more tests did not enter the debugger');
 assertEquals(begin_test_count, end_test_count,
              'one or more tests did not have its result checked');