Fix most warnings and other minor cleanups.

lib/compiler/implementation/dart_backend/backend.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Visitor to gather conservative estimate of functions which
  * can be invoked and classes which can be instantiated.
  */
 class ReachabilityVisitor implements Visitor {
   final Compiler compiler;
@@ skipping 90 matching lines @@
   }
 
   String codegen(WorkItem work) {
     // Traverse AST to populate sets of reachable classes and functions.
     log('codegen(${work.element})');
     FunctionExpression function = work.element.parseNode(compiler);
     function.body.accept(new TraversingVisitor(
         new ReachabilityVisitor(compiler, work.resolutionTree)));
   }
 
-  void processNativeClasses(world, libraries) {}
+  void processNativeClasses(Enqueuer world,
+                            Collection<LibraryElement> libraries) {
+  }
+
   void assembleProgram() {
     compiler.assembledCode = '';
   }
 
   log(String message) => compiler.log('[DartBackend] $message');
 }