Make pub build use barback.

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 'barback_logger.dart';
 import 'errors.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 {
   // TODO(rnystrom): Revise how to track error results. Errors can come from
   // both logs and exceptions. Accumulating them is likely slow and a waste of
   // memory. If we do want to accumulate them, we should at least unify them
   // in a single collection (probably of log entries).
   /// All errors that were thrown during the build.
   final Set<BarbackException> errors;
 
-  /// The number of error log entries that occurred during this build.
-  final int _numErrorLogs;
+  /// `true` if the build succeeded.
+  bool get succeeded => errors.isEmpty;
 
-  /// `true` if the build succeeded.
-  bool get succeeded => errors.isEmpty && _numErrorLogs == 0;
-
-  /// Gets the number of error exceptions and log entries.
-  int get numErrors => errors.length + _numErrorLogs;
-
-  BuildResult(Iterable<BarbackException> errors, this._numErrorLogs)
+  BuildResult(Iterable<BarbackException> errors)
       : errors = flattenAggregateExceptions(errors).toSet();
 
   /// Creates a build result indicating a successful build.
   ///
   /// This equivalent to a build result with no errors.
   BuildResult.success()
-      : this([], 0);
+      : this([]);
+
+  /// Creates a single [BuildResult] that contains all of the errors of
+  /// [results].
+  factory BuildResult.aggregate(Iterable<BuildResult> results) {
+    var errors = unionAll(results.map((result) => result.errors));
+    return new BuildResult(errors);
+  }
 
   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");
   }
 }