Make requiresContextBox a getter as per Siggi's suggestion.

pkg/compiler/lib/src/ssa/builder_kernel.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.
 
 import 'package:kernel/ast.dart' as ir;
 
 import '../closure.dart';
 import '../common.dart';
 import '../common/codegen.dart' show CodegenRegistry;
 import '../common/names.dart';
@@ skipping 323 matching lines @@
       // Provide the parameters to the generative constructor body.
       body.function.positionalParameters.forEach(handleParameter);
       body.function.namedParameters.toList()
         ..sort(namedOrdering)
         ..forEach(handleParameter);
 
       // If there are locals that escape (i.e. mutated in closures), we pass the
       // box to the constructor.
       ClosureAnalysisInfo scopeData = closureDataLookup
           .getClosureAnalysisInfo(constructorElement.resolvedAst.node);
-      if (scopeData.requiresContextBox()) {
+      if (scopeData.requiresContextBox) {
         bodyCallInputs.add(localsHandler.readLocal(scopeData.context));
       }
 
       // Pass type arguments.
       ir.Class currentClass = body.enclosingClass;
       if (backend.rtiNeed.classNeedsRti(_elementMap.getClass(currentClass))) {
         for (ir.DartType typeParameter in currentClass.thisType.typeArguments) {
           HInstruction argument = localsHandler.readLocal(localsHandler
               .getTypeVariableAsLocal(_elementMap.getDartType(typeParameter)
                   as ResolutionTypeVariableType));
@@ skipping 3116 matching lines @@
     enterBlock.setBlockFlow(
         new HTryBlockInformation(
             kernelBuilder.wrapStatementGraph(bodyGraph),
             exception,
             kernelBuilder.wrapStatementGraph(catchGraph),
             kernelBuilder.wrapStatementGraph(finallyGraph)),
         exitBlock);
     kernelBuilder.inTryStatement = previouslyInTryStatement;
   }
 }