Make pub build use barback.

sdk/lib/_internal/pub/lib/src/entrypoint.dart

Index: sdk/lib/_internal/pub/lib/src/entrypoint.dart
diff --git a/sdk/lib/_internal/pub/lib/src/entrypoint.dart b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
index 80b2b530e3bd86228b58b2b4eca9516431708d98..46367fa15710e4101e1c99ac06491d8023242998 100644
--- a/sdk/lib/_internal/pub/lib/src/entrypoint.dart
+++ b/sdk/lib/_internal/pub/lib/src/entrypoint.dart
@@ -192,6 +192,26 @@ class Entrypoint {
     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+  /// Gets dependencies if the lockfile is out of date with respect to the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() {