Make requiresContextBox a getter as per Siggi's suggestion.

pkg/compiler/lib/src/closure.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 'common/names.dart' show Identifiers;
 import 'common/resolution.dart' show ParsingContext, Resolution;
 import 'common/tasks.dart' show CompilerTask;
 import 'common.dart';
 import 'compiler.dart' show Compiler;
 import 'constants/expressions.dart';
@@ skipping 35 matching lines @@
 /// analyzing a closure's characteristics, most commonly used to influence how
 /// code should be generated in SSA builder stage.
 class ClosureAnalysisInfo {
   const ClosureAnalysisInfo();
 
   /// If true, this closure accesses a variable that was defined in an outside
   /// scope and this variable gets modified at some point (sometimes we say that
   /// variable has been "captured"). In this situation, access to this variable
   /// is controlled via a wrapper (box) so that updates to this variable
   /// are done in a way that is in line with Dart's closure rules.
-  bool requiresContextBox() => false;
+  bool get requiresContextBox => false;
 
   /// Accessor to the local environment in which a particular closure node is
   /// executed. This will encapsulate the value of any variables that have been
   /// scoped into this context from outside. This is an accessor to the
   /// contextBox that [requiresContextBox] is testing is required.
   Local get context => null;
 
   /// True if the specified variable has been mutated inside the scope of this
   /// closure.
   bool isCaptured(Local variable) => false;
@@ skipping 524 matching lines @@
 
   // If the scope is attached to a [For] contains the variables that are
   // declared in the initializer of the [For] and that need to be boxed.
   // Otherwise contains the empty List.
   List<Local> boxedLoopVariables = const <Local>[];
 
   ClosureScope(this.boxElement, this.capturedVariables);
 
   Local get context => boxElement;
 
-  bool requiresContextBox() => capturedVariables.keys.isNotEmpty;
+  bool get requiresContextBox => capturedVariables.keys.isNotEmpty;
 
   List<Local> get boxedVariables => boxedLoopVariables;
 
   bool get hasBoxedVariables => !boxedLoopVariables.isEmpty;
 
   bool isCaptured(Local variable) {
     return capturedVariables.containsKey(variable);
   }
 
   void forEachCapturedVariable(
@@ skipping 832 matching lines @@
 ///
 /// Move the below classes to a JS model eventually.
 ///
 abstract class JSEntity implements MemberEntity {
   Local get declaredEntity;
 }
 
 abstract class PrivatelyNamedJSEntity implements JSEntity {
   Entity get rootOfScope;
 }