`prefer-const` rule warns when "let" is needed for scoping

on eslint v2.7.0.

echo 'let foo; const bar = function bar() { return foo; }; foo = 3; console.log(bar());' | ./node_modules/.bin/eslint --stdin --rule 'prefer-const: 2' --no-eslintrc --parser-options=ecmaVersion:6 gives

<text>
  1:54  error  'foo' is never reassigned, use 'const' instead  prefer-const

✖ 1 problem (1 error, 0 warnings)

I’d like it not to warn here, since the let is required to be able to refer to it without violating the no-use-before-define rule. If it does warn, then must I disable one of the rules since they are in conflict here?

[mysticatea]

@nzakas I think so.

[mysticatea]

@lukeapage Thank you for the explanation!

@nzakas As I mentioned at https://github.com/eslint/eslint/issues/5692#issuecomment-203690886, I think I can add an option to ignore variables that are read before its first assignment. I can work on this.