Highlight FieldFormalParameterElement as a field only in declaration.

pkg/analysis_server/lib/src/computer/computer_highlights2.dart

Index: pkg/analysis_server/lib/src/computer/computer_highlights2.dart
diff --git a/pkg/analysis_server/lib/src/computer/computer_highlights2.dart b/pkg/analysis_server/lib/src/computer/computer_highlights2.dart
index bfc9d66570df1e85bdb842f5b027435753038557..7d67c261e07f292f47c9fb1c5458b76ff9e39d0b 100644
--- a/pkg/analysis_server/lib/src/computer/computer_highlights2.dart
+++ b/pkg/analysis_server/lib/src/computer/computer_highlights2.dart
@@ -170,7 +170,9 @@ class DartUnitHighlightsComputer2 {
   bool _addIdentifierRegion_field(SimpleIdentifier node) {
     Element element = node.bestElement;
     if (element is FieldFormalParameterElement) {
-      element = (element as FieldFormalParameterElement).field;
+      if (node.parent is FieldFormalParameter) {
+        element = (element as FieldFormalParameterElement).field;
+      }
     }
     // prepare type
     HighlightRegionType type;