Not compiling to valid ES3 code and emits no warning

[haochi]

TypeScript Version: 2.8.0-insiders.20180320

Search Terms: literal, invalid left-hand side assignment, es3

Code

var a = [] = [1].map(_ => _);
var b = [1].map(_ => _);

Expected behavior:
Compiles to valid ES3 code.

var a = [1].map(function (_) { return _; });
var b = [1].map(function (_) { return _; });

Actual behavior:
Doesn't compile to valid ES3 code and emits no warnings.

var a = [1].map(_ => _);
var b = [1].map(function (_) { return _; });

Playground Link: https://www.typescriptlang.org/play/#src=var%20a%20%3D%20%5B%5D%20%3D%20%5B1%5D.map(_%20%3D%3E%20_)%3B%0D%0Avar%20b%20%3D%20%5B1%5D.map(_%20%3D%3E%20_)%3B%0D%0A

Related Issues:
Can't find any.