Remove unused imports to make analyzer happy

pkg/kernel/lib/transformations/closure/context.dart

 // Copyright (c) 2016, 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.
 
 library kernel.transformations.closure.context;
 
 import '../../ast.dart'
     show
-        Class,
         Expression,
         NullLiteral,
         StringLiteral,
         Throw,
         TreeNode,
         VariableDeclaration,
         VariableGet,
         VariableSet,
         VectorCreation,
         VectorGet,
@@ skipping 214 matching lines @@
       // Item 0 of a context always points to its parent.
       context = new VectorGet(context, 0);
     }
     throw 'Unbound NestedContext.lookup($variable)';
   }
 
   Context toNestedContext([Accessor accessor]) {
     return new NestedContext(converter, accessor ?? this.accessor, variabless);
   }
 }