Use Partial types for Object.assign

[falsandtru]

Fixes #11100

[HerringtonDarkholme]

Thanks for your contribution, but IMHO, let's waiting for rest type (#10727) since it is a better choice.

http://www.typescriptlang.org/play/index.html#src=declare%20function%20assign%3CT%3E(target%3A%20Partial%3CT%3E%2C%20...sources%3A%20Partial%3CT%3E%5B%5D)%3A%20T%3B%0Aassign(%7Ba%3A%20123%7D%2C%20%7Bb%3A%20345%7D)

[falsandtru]

Yep, reverted my deletions. These problems (by my deletions) will be resolved by rest type. However as I said in #11100, I think partial type is needed too. So this pr can merge without waiting to merge #10727.

[sandersn]

The partial type is not right for assign for a couple of reasons.

1. It's not a useful inference source. So function f<T>(pt: Partial<T>): T is essentially equivalent to function f<T>(t: T): T unless the type argument is given explicitly.
2. It's too strict; see @HerringtonDarkholme's example.

Also, the variadic overload should come last, right? Otherwise it will preclude matching other overloads.

I can see three options:

1. Do nothing. Variadic Object.assign continues to be illegal in Typescript, although (hard-coded) variadic spread is now legal.
2. Punt. Make variadic assign take and return any.
3. Be strict. Use T as the type parameter instead of Partial<T>.

[falsandtru]

I see, thanks for your explanation!