Make requiresContextBox a getter as per Siggi's suggestion.

pkg/compiler/lib/src/ssa/builder.dart

 // Copyright (c) 2012, 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 'dart:collection';
 
 import 'package:js_runtime/shared/embedded_names.dart';
 
 import '../closure.dart';
 import '../common.dart';
@@ skipping 1378 matching lines @@
         if (!localsHandler.isBoxed(parameter)) {
           bodyCallInputs.add(localsHandler.readLocal(parameter));
         }
       });
 
       // If there are locals that escape (ie mutated in closures), we
       // pass the box to the constructor.
       // The box must be passed before any type variable.
       ClosureAnalysisInfo scopeData =
           closureDataLookup.getClosureAnalysisInfo(node);
-      if (scopeData.requiresContextBox()) {
+      if (scopeData.requiresContextBox) {
         bodyCallInputs.add(localsHandler.readLocal(scopeData.context));
       }
 
       // Type variables arguments must come after the box (if there is one).
       ClassElement currentClass = constructor.enclosingClass;
       if (rtiNeed.classNeedsRti(currentClass)) {
         // If [currentClass] needs RTI, we add the type variables as
         // parameters of the generative constructor body.
         currentClass.typeVariables
             .forEach((ResolutionTypeVariableType argument) {
@@ skipping 5457 matching lines @@
       this.oldReturnLocal,
       this.oldReturnType,
       this.oldResolvedAst,
       this.oldStack,
       this.oldLocalsHandler,
       this.inTryStatement,
       this.allFunctionsCalledOnce,
       this.oldElementInferenceResults)
       : super(function);
 }