Fix (bogus) warnings related to the use of Function in dart2js.

sdk/lib/_internal/compiler/implementation/enqueue.dart

Index: sdk/lib/_internal/compiler/implementation/enqueue.dart
diff --git a/sdk/lib/_internal/compiler/implementation/enqueue.dart b/sdk/lib/_internal/compiler/implementation/enqueue.dart
index d1ee2cce5f916a20bca6110779e817d54f758fe8..8d24ad2ee56f12254a7bbdd91bbb20756eb26820 100644
--- a/sdk/lib/_internal/compiler/implementation/enqueue.dart
+++ b/sdk/lib/_internal/compiler/implementation/enqueue.dart
@@ -4,6 +4,8 @@
 
 part of dart2js;
 
+typedef ItemCompilationContext ItemCompilationContextCreator();
+
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
   final CodegenEnqueuer codegen;
@@ -71,7 +73,7 @@ class EnqueueTask extends CompilerTask {
 abstract class Enqueuer {
   final String name;
   final Compiler compiler; // TODO(ahe): Remove this dependency.
-  final Function itemCompilationContextCreator;
+  final ItemCompilationContextCreator itemCompilationContextCreator;
   final Map<String, Link<Element>> instanceMembersByName
       = new Map<String, Link<Element>>();
   final Map<String, Link<Element>> instanceFunctionsByName
@@ -85,9 +87,7 @@ abstract class Enqueuer {
 
   bool hasEnqueuedEverything = false;
 
-  Enqueuer(this.name, this.compiler,
-           ItemCompilationContext itemCompilationContextCreator())
-    : this.itemCompilationContextCreator = itemCompilationContextCreator;
+  Enqueuer(this.name, this.compiler, this.itemCompilationContextCreator);
 
   Queue<WorkItem> get queue;