Move barback to a more event-based model.

pkg/barback/test/package_graph/source_test.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 barback.test.package_graph.source_test;
 
 import 'dart:async';
 
 import 'package:barback/barback.dart';
+import 'package:barback/src/utils.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
 main() {
   initConfig();
   test("gets a source asset", () {
     initGraph(["app|foo.txt"]);
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt");
@@ skipping 85 matching lines @@
 
     schedule(() {
       removeSources(["app|foo.txt"]);
       updateSources(["app|foo.txt"]);
     });
 
     expectAsset("app|foo.txt");
     buildShouldSucceed();
   });
 
+  test("reloads an asset that's updated while loading", () {
+    initGraph({"app|foo.txt": "foo"});
+
+    pauseProvider();
+    schedule(() {
+      // The mock provider synchronously loads the value of the assets, so this
+      // will kick off two loads with different values. The second one should
+      // win.
+      updateSources(["app|foo.txt"]);
+      modifyAsset("app|foo.txt", "bar");
+      updateSources(["app|foo.txt"]);
+    });
+
+    resumeProvider();
+    expectAsset("app|foo.txt", "bar");
+    buildShouldSucceed();
+  });
+
   test("restarts a build if a source is updated while sources are loading", () {
     var transformer = new RewriteTransformer("txt", "out");
     initGraph(["app|foo.txt", "app|other.bar"], {"app": [[transformer]]});
 
     // Run the whole graph so all nodes are clean.
     updateSources(["app|foo.txt", "app|other.bar"]);
     expectAsset("app|foo.out", "foo.out");
     expectAsset("app|other.bar");
 
     buildShouldSucceed();
@@ skipping 14 matching lines @@
     resumeProvider();
 
     buildShouldSucceed();
     waitForBuild();
 
     schedule(() {
       expect(transformer.numRuns, equals(2));
     });
   });
 }