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

sdk/lib/_internal/compiler/implementation/enqueue.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.
 
 part of dart2js;
 
+typedef ItemCompilationContext ItemCompilationContextCreator();
+
 class EnqueueTask extends CompilerTask {
   final ResolutionEnqueuer resolution;
   final CodegenEnqueuer codegen;
 
   /// A reverse map from name to *all* elements with that name, not
   /// just instance members of instantiated classes.
   final Map<String, Link<Element>> allElementsByName
       = new Map<String, Link<Element>>();
 
   void ensureAllElementsByName() {
@@ skipping 47 matching lines @@
 
     codegen.nativeEnqueuer = compiler.backend.nativeCodegenEnqueuer(codegen);
     resolution.nativeEnqueuer =
         compiler.backend.nativeResolutionEnqueuer(resolution);
   }
 }
 
 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
       = new Map<String, Link<Element>>();
   final Set<ClassElement> seenClasses = new Set<ClassElement>();
   final Universe universe = new Universe();
 
   bool queueIsClosed = false;
   EnqueueTask task;
   native.NativeEnqueuer nativeEnqueuer;  // Set by EnqueueTask
 
   bool hasEnqueuedEverything = false;
 
-  Enqueuer(this.name, this.compiler,
-           ItemCompilationContext itemCompilationContextCreator())
-    : this.itemCompilationContextCreator = itemCompilationContextCreator;
+  Enqueuer(this.name, this.compiler, this.itemCompilationContextCreator);
 
   Queue<WorkItem> get queue;
 
   /// Returns [:true:] if this enqueuer is the resolution enqueuer.
   bool get isResolutionQueue => false;
 
   /// Returns [:true:] if [member] has been processed by this enqueuer.
   bool isProcessed(Element member);
 
   /**
@@ skipping 629 matching lines @@
     }
     CodegenWorkItem workItem = new CodegenWorkItem(
         element, itemCompilationContextCreator());
     queue.add(workItem);
   }
 
   void _logSpecificSummary(log(message)) {
     log('Compiled ${generatedCode.length} methods.');
   }
 }