Gracefully parse `abstract class` in expression position

[mattpap]

TypeScript Version: 2.4.2 / nightly (2.5.0-dev.20170725)

Code

function f1() {
  return abstract class {}
}

function f2() {
  const x = abstract class {}
  return x
}

function f3() {
  abstract class x {}
  return x
}

Expected behavior:

No errors reported by the compiler.

Actual behavior:

some.ts(2,19): error TS1005: ';' expected.
some.ts(6,22): error TS1005: ',' expected.

[DanielRosenwasser]

We aren't looking to support them (see #4578), but it wouldn't hurt to parse these and provide a useful error.

[canonic-epicure]

Please implement this, simplifies notation, when using mixins.