Add WorkItemBuilder to abstract WorkItem creation from the enqueuers.

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index 3de6d0f6c9df6077c1e3dfeb3f7e98dcfc578e53..40ee92bd7aa4ac59511cfc273428911c0ca97735 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -15,6 +15,7 @@ import 'elements/elements.dart'
         ClassElement,
         Element,
         FunctionElement,
+        MemberElement,
         MixinApplicationElement,
         TypedefElement,
         FieldElement;
@@ -338,7 +339,7 @@ abstract class OpenWorld implements World {
   /// classes that extend or implement it.
   void registerClass(ClassElement cls);
 
-  void registerUsedElement(Element element);
+  void registerUsedElement(MemberElement element);
   void registerTypedef(TypedefElement typedef);
 
   ClosedWorld closeWorld(DiagnosticReporter reporter);