[Suggestion] Warn if user overrides Vue's internal methods

[phanan]

Currently, properties that start with _ or $ are not proxied on the Vue instance because they may conflict with Vue’s internal. The same doesn't apply for methods, leading to problematic cases like this one, where users may (accidentally) define methods which override Vue's internal API.

Suggestion: Though this arguably belongs in userland, it might be helpful of Vue to introduce a console warning (Warn: You're overriding Vue's internal method $emit. Be aware of misbehaviors.) for such cases.

[yyx990803]

Yes - PR welcome.