Remove unnecessary contexts in closure conversion

pkg/kernel/lib/transformations/closure/converter.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.converter;
 
 import '../../ast.dart'
     show
         Arguments,
         Block,
@@ skipping 396 matching lines @@
 
     currentMember = node;
 
     if (node.isInstanceMember) {
       Name tearOffName = tearOffGetterNames[node.name];
       if (tearOffName != null) {
         if (node.isGetter) {
           // We rename the getter to avoid an indirection in most cases.
           Name oldName = node.name;
           node.name = tearOffName;
-          node.reference.canonicalName.unbind();
+          node.canonicalName?.unbind();
           addGetterForwarder(oldName, node);
         } else if (node.kind == ProcedureKind.Method) {
           addTearOffMethod(tearOffName, node);
         }
       }
     }
 
     FunctionNode function = node.function;
     if (function.body != null) {
       setupContextForFunctionBody(function);
@@ skipping 448 matching lines @@
         newClassMembers.add(tearOffMethod);
 
         resetContext();
       });
     } finally {
       currentMember = oldCurrentMember;
       currentMemberFunction = oldCurrentMemberFunction;
     }
   }
 }