Fix most warnings and other minor cleanups.

lib/compiler/implementation/dart_backend/backend.dart

Index: lib/compiler/implementation/dart_backend/backend.dart
diff --git a/lib/compiler/implementation/dart_backend/backend.dart b/lib/compiler/implementation/dart_backend/backend.dart
index ba8f44a9f8bcd9187d9ce1fd12cd86e2e500794b..f547e2d6c6bb0f51086940e2a9524c0067a4d8db 100644
--- a/lib/compiler/implementation/dart_backend/backend.dart
+++ b/lib/compiler/implementation/dart_backend/backend.dart
@@ -108,7 +108,10 @@ class DartBackend extends Backend {
         new ReachabilityVisitor(compiler, work.resolutionTree)));
   }
 
-  void processNativeClasses(world, libraries) {}
+  void processNativeClasses(Enqueuer world,
+                            Collection<LibraryElement> libraries) {
+  }
+
   void assembleProgram() {
     compiler.assembledCode = '';
   }