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[2.7.10] readonly on frozen object causing TypeError: Cannot define property __v_rawToReadonly

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### Version 2.7.10 ### Reproduction link [codesandbox.io](https://codesandbox.io/s/vue-2-7-v-rawtoreadonly-demo-10ix10?file=/src/App.vue) ### Steps to reproduce reason: Readonly function sets new property on object recursively. This may cause this error when meeting frozen variables. Of...

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