Improve error when using mutable object as prop

Summary:
I was trying to add an object property to my ViewManager and got a really opaque error (and I almost thought Objects weren't supported by the codegen) until I realized it expected the object to wrapped in a $ReadOnly type.

Changelog: [Internal] Improved error in codegen

Reviewed By: mdvacca

Differential Revision: D34006557

fbshipit-source-id: b3ab15a40cb66fdcd377f4e68df92060498e8e7f

packages/react-native-codegen/src/parsers/flow/components/props.js

@@ -319,6 +319,10 @@ function getTypeAnnotation(
           `Unsupported union type for "${name}", received "${unionType}"`,
         );
       }
+    case 'ObjectTypeAnnotation':
+      throw new Error(
+        `Cannot use "${type}" type annotation for "${name}": object types must be declared using $ReadOnly<>`,
+      );
     case 'NumberTypeAnnotation':
       throw new Error(
         `Cannot use "${type}" type annotation for "${name}": must use a specific numeric type like Int32, Double, or Float`,