Extract 'DartUnitHighlightsComputer' tests into a separate file.

pkg/analysis_server/lib/src/computer/computer_highlights.dart

 // Copyright (c) 2014, 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 computer.highlights;
 
+import 'package:analysis_server/src/constants.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/element.dart';
 import 'package:analyzer/src/generated/scanner.dart';
 
 
 /**
  * A computer for [HighlightRegion]s in a Dart [CompilationUnit].
  */
 class DartUnitHighlightsComputer {
   final CompilationUnit _unit;
@@ skipping 239 matching lines @@
 
   bool _addIdentifierRegion_typeParameter(SimpleIdentifier node) {
     Element element = node.staticElement;
     if (element is! TypeParameterElement) {
       return false;
     }
     return _addRegion_node(node, HighlightType.TYPE_PARAMETER);
   }
 
   void _addRegion(int offset, int length, HighlightType type) {
-    _regions.add({'offset': offset, 'length': length, 'type': type.name});
+    _regions.add({OFFSET: offset, LENGTH: length, TYPE: type.name});
   }
 
   bool _addRegion_node(AstNode node, HighlightType type) {
     int offset = node.offset;
     int length = node.length;
     _addRegion(offset, length, type);
     return true;
   }
 
   void _addRegion_nodeStart_tokenEnd(AstNode a, Token b, HighlightType type) {
@@ skipping 226 matching lines @@
   static const HighlightType METHOD = const HighlightType('METHOD');
   static const HighlightType METHOD_STATIC = const HighlightType('METHOD_STATIC');
   static const HighlightType PARAMETER = const HighlightType('PARAMETER');
   static const HighlightType SETTER_DECLARATION = const HighlightType('SETTER_DECLARATION');
   static const HighlightType TOP_LEVEL_VARIABLE = const HighlightType('TOP_LEVEL_VARIABLE');
   static const HighlightType TYPE_NAME_DYNAMIC = const HighlightType('TYPE_NAME_DYNAMIC');
   static const HighlightType TYPE_PARAMETER = const HighlightType('TYPE_PARAMETER');
 
   final String name;
 
-  @override
-  String toString() => name;
-
   const HighlightType(this.name);
 }