Support cross-package transforms in barback.

pkg/barback/lib/src/build_result.dart

Index: pkg/barback/lib/src/build_result.dart
diff --git a/pkg/barback/lib/src/build_result.dart b/pkg/barback/lib/src/build_result.dart
new file mode 100644
index 0000000000000000000000000000000000000000..32ee7c7a217113e1feb31dc37619f6d37e9ddd3f
--- /dev/null
+++ b/pkg/barback/lib/src/build_result.dart
@@ -0,0 +1,51 @@
+// 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.build_result;
+
+import 'dart:async';
+
+import 'package:stack_trace/stack_trace.dart';
+
+import 'utils.dart';
+
+/// An event indicating that the cascade has finished building all assets.
+///
+/// A build can end either in success or failure. If there were no errors during
+/// the build, it's considered to be a success; any errors render it a failure,
+/// although individual assets may still have built successfully.
+class BuildResult {
+  /// All errors that occurred during the build.
+  final List errors;
+
+  /// `true` if the build succeeded.
+  bool get succeeded => errors.isEmpty;
+
+  BuildResult(Iterable errors)
+      : errors = errors.toList();
+
+  /// Creates a build result indicating a successful build.
+  ///
+  /// This equivalent to a build result with no errors.
+  BuildResult.success()
+      : this([]);
+
+  String toString() {
+    if (succeeded) return "success";
+
+    return "errors:\n" + errors.map((error) {
+      var stackTrace = getAttachedStackTrace(error);
+      if (stackTrace != null) stackTrace = new Trace.from(stackTrace);
+
+      var msg = new StringBuffer();
+      msg.write(prefixLines(error.toString()));
+      if (stackTrace != null) {
+        msg.write("\n\n");
+        msg.write("Stack trace:\n");
+        msg.write(prefixLines(stackTrace.toString()));
+      }
+      return msg.toString();
+    }).join("\n\n");
+  }
+}