Add WorkItemBuilder to abstract WorkItem creation from the enqueuers.

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 57f3ee73de4c9cb598e4befba9bc6452b3eff66c..962262427862bba0385fc05c7d5bc8f17f5ce64f 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -1761,7 +1761,7 @@ class JavaScriptBackend extends Backend {
     return _closedWorld.hasOnlySubclasses(classElement);
   }
 
-  WorldImpact registerUsedElement(Element element, {bool forResolution}) {
+  WorldImpact registerUsedElement(MemberElement element, {bool forResolution}) {
     WorldImpactBuilderImpl worldImpact = new WorldImpactBuilderImpl();
     if (element == helpers.disableTreeShakingMarker) {
       isTreeShakingDisabled = true;
@@ -2139,7 +2139,7 @@ class JavaScriptBackend extends Backend {
     for (LibraryElement lib in compiler.libraryLoader.libraries) {
       if (lib.isInternalLibrary) continue;
       lib.forEachLocalMember((Element member) {
-        if (!member.isClass &&
+        if (!(member.isClass || member.isTypedef) &&
             resolution.hasBeenProcessed(member) &&
             referencedFromMirrorSystem(member)) {
           reflectableMembers.add(member);