Use LIBRARY_SPECIFIC_UNITS instead of UNITS.

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';
@@ skipping 133 matching lines @@
 
 /**
  * A specific compilation unit in a specific library.
  *
  * This kind of target is associated with information about a compilation unit
  * that differs based on the library that the unit is a part of. For example,
  * the result of resolving a compilation unit depends on the imports, which can
  * change if a single part is included in more than one library.
  */
 class LibrarySpecificUnit implements AnalysisTarget {
+  static const List<LibrarySpecificUnit> EMPTY_LIST =
+      const <LibrarySpecificUnit>[];
+
   /**
    * The defining compilation unit of the library in which the [unit]
    * is analyzed.
    */
   final Source library;
 
   /**
    * The compilation unit which belongs to the [library].
    */
   final Source unit;
@@ skipping 14 matching lines @@
   @override
   bool operator ==(other) {
     return other is LibrarySpecificUnit &&
         other.library == library &&
         other.unit == unit;
   }
 
   @override
   String toString() => '$unit in $library';
 }