Chakra should throw SyntaxError when a variable is already declared inside the "with" scope

Hi,

there is an inconsistency when a variable is declared twice as "var" and "let" inside the "with" scope. Chakra should not accept the redeclaration and indicate an error.

OS: Ubuntu 17.10
Chakra: 1.10.0.0-beta

Step to reproduce:
```
with({}) {
var a;
let a;
}
```

Actual result:
Pass without failures

Expected result:
SyntaxError: Cannot declare a variable twice

V8 and SpiderMonkey raises an exception as expected.