Use `Function` as function type syntax.

pkg/compiler/lib/src/kernel/kernel.dart

Index: pkg/compiler/lib/src/kernel/kernel.dart
diff --git a/pkg/compiler/lib/src/kernel/kernel.dart b/pkg/compiler/lib/src/kernel/kernel.dart
index 53ca7823e1725d958337e109a6e3be1a6c9b3d33..90000a22c76a85c08c3f7b3b1537a2154186b468 100644
--- a/pkg/compiler/lib/src/kernel/kernel.dart
+++ b/pkg/compiler/lib/src/kernel/kernel.dart
@@ -35,11 +35,9 @@ import '../tree/tree.dart' show FunctionExpression, Node;
 import 'constant_visitor.dart';
 import 'kernel_visitor.dart' show IrFunction, KernelVisitor;
 
-typedef void WorkAction();
-
 class WorkItem {
   final Element element;
-  final WorkAction action;
+  final void Function() action;
 
   WorkItem(this.element, this.action);
 }
@@ -80,7 +78,7 @@ class Kernel {
 
   Kernel(this.compiler);
 
-  void addWork(Element element, WorkAction action) {
+  void addWork(Element element, void Function() action) {
     workQueue.addLast(new WorkItem(element, action));
   }