Remove assert that prevents nested scopes closing over save variable

pkg/compiler/lib/src/js_model/closure_visitors.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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../elements/entities.dart';
 import '../kernel/element_map.dart';
 import 'closure.dart';
@@ skipping 248 matching lines @@
     _isInsideClosure = oldIsInsideClosure;
     _currentScopeInfo = oldScopeInfo;
     _executableContext = oldExecutableContext;
 
     // Mark all free variables as captured and expect to encounter them in the
     // outer function.
     Iterable<ir.VariableDeclaration> freeVariables =
         savedScopeInfo.freeVariables;
     assert(freeVariables.isEmpty || savedIsInsideClosure);
     for (ir.VariableDeclaration freeVariable in freeVariables) {
-      assert(!_capturedVariables.contains(freeVariable));
       _capturedVariables.add(freeVariable);
       _markVariableAsUsed(freeVariable);
     }
   }
 
   /// Return true if [variable]'s context is the same as the current executable
   /// context.
   bool _inCurrentContext(ir.VariableDeclaration variable) {
     ir.TreeNode node = variable;
     while (node != _outermostNode && node != _executableContext) {
@@ skipping 35 matching lines @@
               nodeToConvert.isInstanceMember)) {
         return new ThisLocal(_kernelToElementMap.getConstructor(nodeToConvert));
       } else if (nodeToConvert is ir.Procedure &&
           nodeToConvert.isInstanceMember) {
         return new ThisLocal(_kernelToElementMap.getMethod(nodeToConvert));
       }
     }
     return null;
   }
 }