Add WorkItemBuilder to abstract WorkItem creation from the enqueuers.

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 3eee56fa326f44772774f777eca78d63c2d32abb..046573fa6b0a649e80c57ada47d61ca53f279f39 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -769,6 +769,9 @@ abstract class Compiler implements LibraryLoaderListener {
         cls.ensureResolved(resolution);
         cls.forEachLocalMember(registerStaticUse);
         impactBuilder.registerTypeUse(new TypeUse.instantiation(cls.rawType));
+      } else if (element.isTypedef) {

[Siggi Cherem (dart-lang), 2016/12/20 21:21:47]

I feel I missed something: why typedefs are now handled differently? I didn't
find the relation between that and adding the workitem builder abstration

[Johnni Winther, 2016/12/21 10:19:37]

Typedef should never have been added to the work-list. The work-item created for
them would effectively call `ensureResolved` as below. They were the last reason
why we couldn't state that all enqueued entities were MemberEntity; which is why
the change is in this CL.

+        TypedefElement typdef = element;
+        typdef.ensureResolved(resolution);
       } else {
         registerStaticUse(element);
       }