Fix for highlighting 'abstract' in ClassTypeAlias.

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 e0d7337f1aee1b0867d9441630eb1167ce940481..20f618fcf397e28466b8cf3dbedd170f24b2752b 100644
--- a/pkg/analysis_server/lib/src/computer/computer_highlights.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_highlights.dart
@@ -383,6 +383,14 @@ class _DartUnitHighlightsComputerVisitor extends RecursiveAstVisitor<Object> {
   }
 
   @override
+  Object visitClassTypeAlias(ClassTypeAlias node) {
+    computer._addRegion_token(
+        node.abstractKeyword,
+        HighlightRegionType.BUILT_IN);
+    return super.visitClassTypeAlias(node);
+  }
+
+  @override
   Object visitConstructorDeclaration(ConstructorDeclaration node) {
     computer._addRegion_token(
         node.externalKeyword,