Refactor Library/CompilationUnit and how we define local Scope.

lib/compiler/implementation/world.dart

Index: lib/compiler/implementation/world.dart
diff --git a/lib/compiler/implementation/world.dart b/lib/compiler/implementation/world.dart
index 7254d7a7f68e87537c73a78fe71b319e493d80c8..867812263059829ed3d92a5478393b8bd542f9f1 100644
--- a/lib/compiler/implementation/world.dart
+++ b/lib/compiler/implementation/world.dart
@@ -19,10 +19,7 @@ class World {
     }
 
     libraries.forEach((LibraryElement library) {
-      for (Link<Element> link = library.topLevelElements;
-           !link.isEmpty();
-           link = link.tail) {
-        Element element = link.head;
+      for (Element element in library.localMembers) {
         if (!element.isClass()) continue;
         ClassElement cls = element;
         compiler.resolveClass(cls);