Type Guards no longer work inside a nested closure.

[david-driscoll]

TypeScript Version:
nightly (1.9.0-dev.20160217)

Code

interface TypeGuard {
  guard: boolean;
}
function isTypeGuard(value: any): value is TypeGuard {
  return value.guard;
}

let a: { notGuard: boolean } | TypeGuard;
if (isTypeGuard(a)) {
  const b: TypeGuard = a;
  let c: TypeGuard
  (() => c = a)();
}

Expected behavior:
The guarded type should be guarded within any nested closures.

Actual behavior:
The guarded type is not guarded in any nested closures. This has previously been working correctly.

Example outputs...

This is impacting ReactiveX/rxjs#1697

cc @alexeagle

[malibuzios]

Related: #7719, #7739, #8367, #8541

I believe this has been described as a breaking change in #7662

[david-driscoll]

Alright makes sense for a major rev. Thanks!