Remove unnecessary contexts in closure conversion

pkg/kernel/lib/transformations/closure/converter.dart

Index: pkg/kernel/lib/transformations/closure/converter.dart
diff --git a/pkg/kernel/lib/transformations/closure/converter.dart b/pkg/kernel/lib/transformations/closure/converter.dart
index 93c0d71036e24a27404856632a8833845dabb9e1..e1865f7fd6d90a6c04a8e0dbeba37456490f612f 100644
--- a/pkg/kernel/lib/transformations/closure/converter.dart
+++ b/pkg/kernel/lib/transformations/closure/converter.dart
@@ -414,7 +414,7 @@ class ClosureConverter extends Transformer {
           // 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);