Mixin data is lost when a previous mixin watches that data

[rashfael]

Version

3.0.0-beta.3

Reproduction link

https://jsfiddle.net/ek7bjy9q/13/

Steps to reproduce

None.

What is expected?

Text should read mixin1 mixin3.

What is actually happening?

Text reads mixin1 and vue.esm-browser.js:4685 [Vue warn]: Property "mixin3Data" was accessed during render but is not defined on instance. is printed in the console.

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If the mixin order is [mixin1, mixin3, mixin2] instead, the text is displayed correctly.