Add instrumentation for tracking how long it takes barback to start.

sdk/lib/_internal/pub/lib/src/barback/barback_server.dart

 // Copyright (c) 2013, 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.
 
 library pub.barback.server;
 
 import 'dart:async';
 import 'dart:io';
 
 import 'package:barback/barback.dart';
@@ skipping 83 matching lines @@
       return notFound(request, error: ex.message);
     }
 
     // See if the asset should be blocked.
     if (allowAsset != null && !allowAsset(id)) {
       return notFound(request,
           error: "Asset $id is not available in this configuration.",
           asset: id);
     }
 
-    logRequest(request, "Loading $id");
     return environment.barback.getAssetById(id).then((result) {
-      logRequest(request, "getAssetById($id) returned");
       return result;
     }).then((asset) => _serveAsset(request, asset)).catchError((error, trace) {
       if (error is! AssetNotFoundException) throw error;
       return environment.barback.getAssetById(id.addExtension("/index.html"))
           .then((asset) {
         if (request.url.path.endsWith('/')) return _serveAsset(request, asset);
 
         // We only want to serve index.html if the URL explicitly ends in a
         // slash. For other URLs, we redirect to one with the slash added to
         // implicitly support that too. This follows Apache's behavior.
@@ skipping 66 matching lines @@
   bool get isSuccess => error == null;
 
   /// Whether the request was served unsuccessfully.
   bool get isFailure => !isSuccess;
 
   BarbackServerResult._success(this.url, this.id)
       : error = null;
 
   BarbackServerResult._failure(this.url, this.id, this.error);
 }