Fix(Symbol): Conflictive polyfill

[giautm]

the function getSymbol() can return a different result in the different context if there was a polyfill.

I ran into this exception when used zen-observable with firebase@>5.0.3.

zen-observable/src/Observable.js

Line 441 in b9b7fa7

throw new TypeError(x + ' is not observable');

Because this line returns a wrong result:

zen-observable/src/Observable.js

Line 399 in b9b7fa7

let method = getMethod(x, getSymbol('observable'));

It's different with the key defined at:

zen-observable/src/Observable.js

Line 391 in b9b7fa7

[getSymbol('observable')]() { return this }

This PR is for internal changes, and can make other developer can live easier. 😄

Linked firebase issues: firebase/firebase-js-sdk#871 (comment)

[giautm]

@zenparsing : Sorry, can you review this PR?

[zenparsing]

Published in 0.8.10. Thanks!