Catch another exception earlier in the code

Change-Id: Ia9fdca760c4c0838cdb6bb31c00f296fbf21cbac
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106102
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>
Auto-Submit: Brian Wilkerson <brianwilkerson@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>

Author: bwilkerson

pkg/nnbd_migration/lib/src/graph_builder.dart

@@ -530,6 +530,10 @@ $stackTrace''');
       if (typeArguments == null) {
         var instantiatedType =
             _variables.decoratedTypeAnnotation(_source, typeName);
+        if (instantiatedType == null) {
+          throw new StateError('No type annotation for type name '
+              '${typeName.toSource()}, offset=${typeName.offset}');
+        }
         for (int i = 0; i < instantiatedType.typeArguments.length; i++) {
           _unionDecoratedTypes(
               instantiatedType.typeArguments[i],