Handle synchronous asset load errors in barback.

pkg/barback/test/package_graph/errors_test.dart

Index: pkg/barback/test/package_graph/errors_test.dart
diff --git a/pkg/barback/test/package_graph/errors_test.dart b/pkg/barback/test/package_graph/errors_test.dart
index 02a7675d47d9d95e0ada223405bae4fae4e43ce2..1b578a57aca333d19414fc85c659f420765d8444 100644
--- a/pkg/barback/test/package_graph/errors_test.dart
+++ b/pkg/barback/test/package_graph/errors_test.dart
@@ -145,6 +145,22 @@ main() {
     ]);
   });
 
+  test("a synchronous error loading an asset removes the asset from the graph",
+      () {
+    initGraph(["app|foo.txt"], {"app": [
+      // Have a dummy transformer so that barback at least tries to load the
+      // asset.
+      [new RewriteTransformer("a", "b")]
+    ]});
+
+    setAssetError("app|foo.txt", async: false);
+    updateSources(["app|foo.txt"]);
+    expectNoAsset("app|foo.txt");
+    buildShouldFail([
+      isAssetLoadException("app|foo.txt", isMockLoadException("app|foo.txt"))
+    ]);
+  });
+
   test("an asset isn't passed through a transformer with an error", () {
     initGraph(["app|foo.txt"], {"app": [[new BadTransformer([])]]});