Add logging for work items being processed.

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 {
   visitBlock(Block node) {}
@@ skipping 73 matching lines @@
 class DartBackend extends Backend {
   final ReachabilityVisitor reachabilityVisitor;
   final List<CompilerTask> tasks = const <CompilerTask>[];
 
   DartBackend(Compiler compiler)
       : reachabilityVisitor = new ReachabilityVisitor(),
         super(compiler);
 
   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(reachabilityVisitor));
   }
 
   void processNativeClasses(world, libraries) {}
   void assembleProgram() {
     compiler.assembledCode = '';
   }
+
+  log(String message) => compiler.log('[DartBackend] $message');
 }