Add offsets to the `== null` calls inserted by null-aware desugaring.

pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart

Index: pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
diff --git a/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart b/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
index e62ede56ac3c587c29db9965d08421340c353473..f837e4f50ad2ac1e08556f5e211f9eccf8096b7d 100644
--- a/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
+++ b/pkg/front_end/lib/src/fasta/testing/validating_instrumentation.dart
@@ -156,6 +156,9 @@ class ValidatingInstrumentation implements Instrumentation {
   void record(
       Uri uri, int offset, String property, InstrumentationValue value) {
     uri = Uri.base.resolveUri(uri);
+    if (offset == -1) {
+      throw _formatProblem(uri, 0, 'No offset for $property=$value', null);
+    }
     var expectationsForUri = _unsatisfiedExpectations[uri];
     if (expectationsForUri == null) return;
     var expectationsAtOffset = expectationsForUri[offset];