Make [Barback.getAllAssets] work when called synchronously.

pkg/barback/test/package_graph/get_all_assets_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.barback_test;
 
+import 'dart:async';
+
+import 'package:barback/barback.dart';
 import 'package:scheduled_test/scheduled_test.dart';
 
 import '../utils.dart';
 
 main() {
   initConfig();
 
   test("gets all source assets", () {
     initGraph(["app|a.txt", "app|b.txt", "app|c.txt"]);
     updateSources(["app|a.txt", "app|b.txt", "app|c.txt"]);
@@ skipping 50 matching lines @@
         new BadTransformer(["app|foo.out2"])
       ]
     ]});
 
     updateSources(["app|foo.txt"]);
     expectAllAssetsShouldFail(isAggregateException([
       isTransformerException(equals(BadTransformer.ERROR)),
       isTransformerException(equals(BadTransformer.ERROR))
     ]));
   });
+
+  // Regression test.
+  test("getAllAssets() is called synchronously after after initializing "
+      "barback", () {
+    var provider = new MockProvider({
+      "app|a.txt": "a",
+      "app|b.txt": "b",
+      "app|c.txt": "c"
+    });
+    var barback = new Barback(provider);
+    barback.updateSources([
+      new AssetId.parse("app|a.txt"),
+      new AssetId.parse("app|b.txt"),
+      new AssetId.parse("app|c.txt")
+    ]);
+
+    expect(barback.getAllAssets().then((assets) {
+      return Future.wait(assets.map((asset) => asset.readAsString()));
+    }), completion(unorderedEquals(["a", "b", "c"])));
+  });
 }