Migration: add an option to print just exception nodes rather than fu…

…ll stack traces.

This makes it easier to find a short reproducible test case when
investigating a migration tool exception.

Change-Id: I13adf3e733d5cbbad58c9649420f7fdd072e4bdf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115840
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/nnbd_migration/tool/trial_migration.dart

@@ -78,6 +78,11 @@ main() async {
 /// if its category contains the string.
 const String categoryOfInterest = null;
 
+/// Set this to `true` to cause just the exception nodes to be printed when
+/// `categoryOfInterest` is non-null.  Set this to `false` to cause the full
+/// stack trace to be printed.
+const bool printExceptionNodeOnly = false;
+
 class _Listener implements NullabilityMigrationListener {
   final groupedExceptions = <String, List<String>>{};
 
@@ -131,7 +136,11 @@ Exception $exception
 $stackTrace
 ''';
     if (categoryOfInterest != null && category.contains(categoryOfInterest)) {
-      print(detail);
+      if (printExceptionNodeOnly) {
+        print('$node');
+      } else {
+        print(detail);
+      }
     }
     (groupedExceptions[category] ??= []).add(detail);
     ++numExceptions;