Add more metadata to non-programmatic barback exceptions.

pkg/barback/lib/src/build_result.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.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;
+  final Set<BarbackException> errors;
 
   /// `true` if the build succeeded.
   bool get succeeded => errors.isEmpty;
 
-  BuildResult(Iterable errors)
-      : errors = errors.toList();
+  BuildResult(Iterable<BarbackException> errors)
+      : errors = errors.toSet();
 
   /// 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");
   }
 }