Vue props throw incomprehensible error when passing variables starting with symbols

[mcskeeled]

Version

2.5.17-beta.0

Reproduction link

https://codesandbox.io/s/r0lr534k94

Steps to reproduce

Open the console and see the error:

proxyConsole.js:72 [Vue warn]: Property or method "$_testMixin_testData" is not defined on the instance but referenced during render. Make sure that this property is reactive, either in the data option, or for class-based components, by initializing the property. See: https://vuejs.org/v2/guide/reactivity.html#Declaring-Reactive-Properties.

found in

---> <TestMixin> at /src/App.vue
       <Root>

What is expected?

It's expected for the prop to be defined

What is actually happening?

The prop is undefined if the variable contains symbols in the beginning

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I am not completely sure if this is a bug or intended, since I think it is a bug I opened this issue, yet if this is intended the error shown should be giving more details, it took me some time to understand what was the error because it isn't clear just from the error message.

[Justineo]

See https://vuejs.org/v2/api/#data:

Properties that start with _ or $ will not be proxied on the Vue instance because they may conflict with Vue’s internal properties and API methods. You will have to access them as vm.$data._property.

[mcskeeled]

I understand that, yet I was following the style guide for mixins:
https://vuejs.org/v2/style-guide/#Private-property-names-essential

And still the error message is not comprehensible, the developers should be able to fix this problem without reading the whole docummentation, in other words I think a better error message would be more appropriate.