Allow trailing commas in type arguments

TypeScript Version: 2.7.1

Search Terms: trailing comma template literal types generic

Code

class FooClass<
	A,
	B,
	C,
> {
	a: A;
	b: B;
	c: C;
}

const instance = new FooClass<
	boolean,
	number,
	string, // [ts] Trailing comma not allowed.
	>();

Expected behavior:

No errors as of #16152's resolution.

Actual behavior:

Trailing comma not allowed.

Playground Link: http://www.typescriptlang.org/play/#src=class%20FooClass%3C%0D%0A%09A%2C%0D%0A%09B%2C%0D%0A%09C%2C%0D%0A%3E%20%7B%0D%0A%09a%3A%20A%3B%0D%0A%09b%3A%20B%3B%0D%0A%09c%3A%20C%3B%0D%0A%7D%0D%0A%0D%0Aconst%20instance%20%3D%20new%20FooClass%3C%0D%0A%09boolean%2C%0D%0A%09number%2C%0D%0A%09string%2C%20%2F%2F%20%5Bts%5D%20Trailing%20comma%20not%20allowed.%0D%0A%09%3E()%3B

Related Issues:

#16152. @samal84 mentioned something like this but I couldn't parse what they meant without a code sample (is this it?). I'll take I can, if this is approved, take a stab at removing this error too.

Start by reproducing the provided FooClass example with typescript@next and review the related discussion in #16152. The work is done when trailing commas in type arguments no longer produce an error and coverage exists for the shown case.