Redeclaring 'let' by 'var' or 'var' by 'let' is an error according to ES6 spec, should be a TS compiler error

[zhuravlikjb]

https://people.mozilla.org/~jorendorff/es6-draft.html#sec-scripts-static-semantics-early-errors
https://people.mozilla.org/~jorendorff/es6-draft.html#sec-block-static-semantics-early-errors

• It is a Syntax Error if any element of the LexicallyDeclaredNames of StatementList also occurs in the VarDeclaredNames of StatementList.

function do_something() {
    let foo;
    var foo; // it should be an error
}

Seems that it should be a TS compiler error, too.

p.s.: By the way, Visual Studio's JS editor properly highlights it as an error.