Make requiresContextBox a getter as per Siggi's suggestion.

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

Index: pkg/compiler/lib/src/ssa/locals_handler.dart
diff --git a/pkg/compiler/lib/src/ssa/locals_handler.dart b/pkg/compiler/lib/src/ssa/locals_handler.dart
index 5c350fe9d5aa2a307affa0d58f474b414665907d..d1c433b499243b95238e6e85dc6216e7e45a7a46 100644
--- a/pkg/compiler/lib/src/ssa/locals_handler.dart
+++ b/pkg/compiler/lib/src/ssa/locals_handler.dart
@@ -140,7 +140,7 @@ class LocalsHandler {
       {bool forGenerativeConstructorBody: false}) {
     // See if any variable in the top-scope of the function is captured. If yes
     // we need to create a box-object.
-    if (!closureInfo.requiresContextBox()) return;
+    if (!closureInfo.requiresContextBox) return;
     HInstruction box;
     // The scope has captured variables.
     if (forGenerativeConstructorBody) {