Make pub build use barback.

sdk/lib/_internal/pub/lib/src/entrypoint.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.
 
 library pub.entrypoint;
 
 import 'dart:async';
 
 import 'package:path/path.dart' as path;
 
@@ skipping 174 matching lines @@
 
       return true;
     }
 
     if (!root.dependencies.every(checkDependency)) return false;
     if (!root.devDependencies.every(checkDependency)) return false;
 
     return true;
   }
 
+  /// Gets dependencies if the lockfile is out of date with respect to the
+  /// pubspec.
+  Future ensureLockFileIsUpToDate() {
+    return new Future.sync(() {
+      if (isLockFileUpToDate()) return;
+
+      if (lockFileExists) {
+        log.message(
+            "Your pubspec has changed, so we need to update your lockfile:");
+      } else {
+        log.message(
+            "You don't have a lockfile, so we need to generate that:");
+      }
+
+      return getDependencies().then((_) {
+        log.message("Got dependencies!");
+      });
+    });
+  }
+
   /// Loads the package graph for the application and all of its transitive
   /// dependencies.
   Future<PackageGraph> loadPackageGraph() {
     return Future.wait(loadLockFile().packages.values.map((id) {
       var source = cache.sources[id.source];
       return source.getDirectory(id)
           .then((dir) => new Package.load(id.name, dir, cache.sources));
     })).then((packages) {
       var packageMap = <String, Package>{};
       for (var package in packages) {
@@ skipping 113 matching lines @@
         // Skip directories and broken symlinks.
         if (!fileExists(file)) return false;
 
         var relative = path.relative(file, from: beneath);
         if (_BLACKLISTED_FILES.contains(path.basename(relative))) return false;
         return !path.split(relative).any(_BLACKLISTED_DIRS.contains);
       }).toList();
     });
   }
 }