Allow decorators in ES3

[rbuckton]

This change modifies the checker and the emitter to support the use of decorators when targeting ES3. In addition to removing the ES5-specific use of reduceRight from the __decorate helper, this change also inlines the calls to Object.getOwnPropertyDescriptor and Object.defineProperty in a backwards-compatible fashion that allows us to clean up the emit for ES5 and later by removing various repetitive calls to the aforementioned Object methods.

Addresses #4681.

[DanielRosenwasser]

I'm not sure if I'm okay with dropping the error message - I think that decorators that won't properly work in ES3 should get an error.

[rbuckton]

@DanielRosenwasser Are you talking about a specific error? Part of this change is to have the emit for method decorators in ES3 not include the property descriptor, just as with decorators on property declarations.

[mhegazy]

but this will have a different behavior for ES3/ES5 for method decorators with no errors.

[DanielRosenwasser]

Yes, if method decorators won't work properly, then you should warn users that method decorators are only supported in ES5 or higher.

[rbuckton]

@DanielRosenwasser, In this change method decorators will work in the same way that decorators for property declarations work. No descriptor will be provided nor accepted. There will be no error.

[rbuckton]

@mhegazy Any further concerns/discussion with this PR?

[mhegazy]

👍
I am still not sure about the error though

[rbuckton]

The error that I removed no longer applies. The only difference between decorators in ES3 and ES5 is that ES3 decorators cannot take and don't observe a descriptor.

[DanielRosenwasser]

@rbuckton I'll need to ask a few questions about this offline.