Check whether type parameters are loaded before emitting

lib/src/codegen/js_codegen.dart

Index: lib/src/codegen/js_codegen.dart
diff --git a/lib/src/codegen/js_codegen.dart b/lib/src/codegen/js_codegen.dart
index 2a85f77cae73b54bea1a75ceb03ebe27cbfcd755..9c06502914ee71446804b5a43349068c013a1c57 100644
--- a/lib/src/codegen/js_codegen.dart
+++ b/lib/src/codegen/js_codegen.dart
@@ -993,6 +993,9 @@ class JSCodegenVisitor extends GeneralizingAstVisitor with ConversionVisitor {
           type.normalParameterTypes.every(_typeIsLoaded));
     }
     if (type.isDynamic || type.isVoid || type.isBottom) return true;
+    if (type is ParameterizedType && !type.typeArguments.every(_typeIsLoaded)) {
+      return false;
+    }
     return _loader.isLoaded(type.element);
   }