Fix a barback bug that triggered when an asset was removed.

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: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");
+    buildShouldSucceed();
   });
 
   test("doesn't get an unknown source", () {
     initGraph();
     expectNoAsset("app|unknown.txt");
   });
 
   test("doesn't get an unprovided source", () {
     initGraph();
     updateSources(["app|unknown.txt"]);
@@ skipping 12 matching lines @@
     expectNoAsset("app|foo.txt");
   });
 
   test("gets a source asset if not transformed", () {
     initGraph(["app|foo.txt"], {"app": [
       [new RewriteTransformer("nottxt", "whatever")]
     ]});
 
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt");
+    buildShouldSucceed();
   });
 
   test("doesn't get a removed source", () {
     initGraph(["app|foo.txt"]);
 
     updateSources(["app|foo.txt"]);
     expectAsset("app|foo.txt");
+    buildShouldSucceed();
 
     schedule(() {
       removeSources(["app|foo.txt"]);
     });
 
     expectNoAsset("app|foo.txt");
+    buildShouldSucceed();
   });
 
   test("collapses redundant updates", () {
     var transformer = new RewriteTransformer("blub", "blab");
     initGraph(["app|foo.blub"], {"app": [[transformer]]});
 
     schedule(() {
       // Make a bunch of synchronous update calls.
       updateSources(["app|foo.blub"]);
       updateSources(["app|foo.blub"]);
       updateSources(["app|foo.blub"]);
       updateSources(["app|foo.blub"]);
     });
 
     expectAsset("app|foo.blab", "foo.blab");
+    buildShouldSucceed();
 
     schedule(() {
       expect(transformer.numRuns, equals(1));
     });
   });
 
   test("a removal cancels out an update", () {
     initGraph(["app|foo.txt"]);
 
     schedule(() {
       updateSources(["app|foo.txt"]);
       removeSources(["app|foo.txt"]);
     });
 
     expectNoAsset("app|foo.txt");
+    buildShouldSucceed();
   });
 
   test("an update cancels out a removal", () {
     initGraph(["app|foo.txt"]);
 
     schedule(() {
       removeSources(["app|foo.txt"]);
       updateSources(["app|foo.txt"]);
     });
 
     expectAsset("app|foo.txt");
+    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");
@@ skipping 16 matching lines @@
     resumeProvider();
 
     buildShouldSucceed();
     waitForBuild();
 
     schedule(() {
       expect(transformer.numRuns, equals(2));
     });
   });
 }