Use generic return types for frequently-casted library functions

[RyanCavanaugh]

There are a handful of functions from lib.d.ts which are typically followed by a type assertion

const x = <HTMLDivElement>document.getElementById('zzz');

With the new inference changes, we can write a new overload

getElementById<T extends Element>(id: string): T

so that you can write the cleaner version

const x: HTMLDivElement = document.getElementById('zzz');

TODO: Figure out which functions these apply to

[gcnew]

Such generics will give a false sense of security. IMO, casts are what's appropriate there as they convey the "I know better" assertion.

[zspitz]

The following from the DefinitelyTyped readme is relevant:

Common mistakes

...

• getMeAT<T>(): T:
  If a type parameter does not appear in the types of any parameters, you don't really have a generic function, you just have a disguised type assertion.
  Prefer to use a real type assertion, e.g. getMeAT() as number.
  Example where a type parameter is acceptable: function id<T>(value: T): T;.
  Example where it is not acceptable: function parseJson<T>(json: string): T;.
  Exception: new Map<string, number>() is OK.

[mhegazy]

Though convenient, it is clearly unsafe. users who would want to do that can do it themselves as overloads.