Default parameters should subtract undefined from internal function signature

[blakeembrey]

TypeScript Version: 2.1.4

Code

function f (offset: number) {}

let x = (offset = 0) => {
  f(offset)
}

x = function (offset = 0) {
  f(offset)
}

{
  "compilerOptions": {
    "target": "es5",
    "outDir": "dist",
    "module": "commonjs",
    "noImplicitAny": true,
    "strictNullChecks": true
  }
}

Expected behavior: No error.

Actual behavior:

index.ts(8,5): error TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.

I believe this has actually been an issue in previous releases, but 2.1 has made it apparent in other argument positions. I just tested 2.0 and can reproduce the above sample, but it doesn't fail on 2.0 with (x: string, y = 0) whereas 2.1 does.

[mhegazy]

Looks like the flip side of #11900