Rest parameter fails to compile with `strictNullChecks` and `target es6`

[seanchas116]

TypeScript Version:

1.9.0-dev.20160502

Code

const xs = [1,2,3];
const ys = [4,5];
xs.push(...ys);

Compiled with tsc --target es6 --strictNullChecks test.ts

Expected behavior:

Compile without errors (without --target es6 option, the code compiles successfully)

Actual behavior:

Errors are emitted:

test.ts(3,9): error TS2345: Argument of type 'number | undefined' is not assignable to parameter of type 'number'.
  Type 'undefined' is not assignable to type 'number'.

Seems like values are treated nullable. (similar to #8357)

[mhegazy]

indeed a duplicate of #8357