[cfe] Don't show context on type argument issues if location is null

Closes #84212.

Bug: flutter/flutter#84212
Change-Id: I4138b65988947396c619a4a4fa5044aac63531cf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/203503
Reviewed-by: Johnni Winther <[email protected]>
Commit-Queue: Dmitry Stefantsov <[email protected]>

Author: Dmitry Stefantsov

pkg/front_end/lib/src/fasta/source/source_library_builder.dart

@@ -3577,7 +3577,7 @@ class SourceLibraryBuilder extends LibraryBuilderImpl {
     // limitation of Kernel.
     if (typeParameter != null &&
         typeParameter.fileOffset != -1 &&
-        typeParameter.parent != null) {
+        typeParameter.location != null) {
       // It looks like when parameters come from patch files, they don't
       // have a reportable location.
       (context ??= <LocatedMessage>[]).add(