Get computeError working

pkg/analyzer/lib/task/dart.dart

 // Copyright (c) 2015, 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 analyzer.task.dart;
 
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/error.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 import 'package:analyzer/src/generated/source.dart';
 import 'package:analyzer/src/generated/utilities_general.dart';
 import 'package:analyzer/src/task/dart.dart';
 import 'package:analyzer/task/model.dart';
 
 /**
  * The analysis errors associated with a target.
  *
- * The value combines errors represented by multiple other results.
+ * The result is only available for targets representing a Dart compilation unit.
  */
-// TODO(brianwilkerson) If we want to associate errors with targets smaller than
-// a file, we will need other contribution points to collect them. In which case
-// we might want to rename this and/or document that it applies to files.
-final CompositeResultDescriptor<List<AnalysisError>> DART_ERRORS =
-    new CompositeResultDescriptor<List<AnalysisError>>('DART_ERRORS');
+final ResultDescriptor<List<AnalysisError>> DART_ERRORS =
+    new ResultDescriptor<List<AnalysisError>>(
+        'DART_ERRORS', AnalysisError.NO_ERRORS);
 
 /**
  * The sources of the libraries that are explicitly imported into a library.
  *
  * The list will be empty if there are no explicit imports, but will not be
  * `null`.
  *
  * The result is only available for targets representing a Dart library.
  */
 final ListResultDescriptor<Source> EXPLICITLY_IMPORTED_LIBRARIES =
@@ skipping 137 matching lines @@
   @override
   bool operator ==(other) {
     return other is LibrarySpecificUnit &&
         other.library == library &&
         other.unit == unit;
   }
 
   @override
   String toString() => '$unit in $library';
 }