Ensure that there is a stack trace when reporting a server error

pkg/analysis_server/lib/src/analysis_server.dart

Index: pkg/analysis_server/lib/src/analysis_server.dart
diff --git a/pkg/analysis_server/lib/src/analysis_server.dart b/pkg/analysis_server/lib/src/analysis_server.dart
index e71ac0499f1b54677cc6ad7e97e5e7381bed4891..c88b3588e469b901e052f7d17e9685630db1e475 100644
--- a/pkg/analysis_server/lib/src/analysis_server.dart
+++ b/pkg/analysis_server/lib/src/analysis_server.dart
@@ -867,7 +867,15 @@ class AnalysisServer {
     if (stackTrace != null) {
       stackTraceString = stackTrace.toString();
     } else {
-      stackTraceString = 'null stackTrace';
+      try {
+        throw 'ignored';
+      } catch (ignored, stackTrace) {
+        stackTraceString = stackTrace.toString();
+      }
+      if (stackTraceString == null) {
+        // This code should be unreachable.
+        stackTraceString = 'null stackTrace';
+      }
     }
     // send the notification
     channel.sendNotification(