Fix assign polyfill

[mydea]

In #19649 assign was re-written to just pass through to Object.assign. However, the implementation is not ideal - it assigns all the arguments, which includes the target itself. This can lead to problems, e.g. in ember-test-helpers when you have an implementation like this: assign(event, options), it will end up as Object.assign(event, event, options), and complain that we try to set read-only attributes on event.

See: emberjs/ember-test-helpers#1077

[rwjblue]

Thank you!!!

[stefanpenner]