don't use instanceof operator

[amir-arad]

In my experience there's a bug when you have more than one copy of Mobx in your module system.
The getAtom function checks for atoms using instanceof javascript operator, which uses nominal typings (equiv. to thing.__proto__.constructor === BaseAtom) instead of using structural / duck typing (typeof thing.reportObserved === 'function' && typeof thing.reportChanged === 'function').

so an atom created by mobx A is not recognised by mobx B.

The rationale here is, when you have multiple instances of the Mobx module, you have multiple instances of the module's functions, with no pointer equality between the two otherwise matching functions. Effectively makes the constructors themselves part of a module's state (something we wish to avoid, judging by the global singleton GlobalState).
This can explain the error message in #570 when using npm 2 or some other modules setup that did not go through aggressive tree-shaking.

[amir-arad]

I'd implement a type guard for that type check, It improves code readability and is well suited for runtime contract checks.