Fix library-scope expression evaluation. (#18851)

This also improves diagnostic in case of internal compiler errors.

packages/flutter_tools/lib/src/vmservice.dart

diff --git a/packages/flutter_tools/lib/src/vmservice.dart b/packages/flutter_tools/lib/src/vmservice.dart
index c33beb0..9547e34 100644
--- a/packages/flutter_tools/lib/src/vmservice.dart
+++ b/packages/flutter_tools/lib/src/vmservice.dart
@@ -163,7 +163,7 @@
         final List<String> definitions = params['definitions'].asList;
         final List<String> typeDefinitions = params['typeDefinitions'].asList;
         final String libraryUri = params['libraryUri'].asString;
-        final String klass = params['klass'] != null ? params['klass'].asString : null;
+        final String klass = params['klass'].exists ? params['klass'].asString : null;
         final bool isStatic = params['isStatic'].asBoolOr(false);
 
         try {