Extract 'DartUnitHighlightsComputer' tests into a separate file.

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

Index: pkg/analysis_server/lib/src/computer/computer_highlights.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_highlights.dart b/pkg/analysis_server/lib/src/computer/computer_highlights.dart
index 06a3db494ed9eaa380d2cc591fa12631b20a90c8..f1cf201aa37da4881a1b8072d37020963422153e 100644
--- a/pkg/analysis_server/lib/src/computer/computer_highlights.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_highlights.dart
@@ -4,6 +4,7 @@
 
 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';
@@ -263,7 +264,7 @@ class DartUnitHighlightsComputer {
   }
 
   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) {
@@ -510,8 +511,5 @@ class HighlightType {
 
   final String name;
 
-  @override
-  String toString() => name;
-
   const HighlightType(this.name);
 }