While working on #6805, I noticed that many variables were being used
before they were declared. Technically this worked fine in practice
because we were using the `transform-es2015-block-scoping` Babel plugin,
which transforms `let` and `const` to `var`, which is hoisted. However,
after removing that Babel transformation, many things broke.
All instances of variables or classes being used before declared have
been fixed.
The `no-use-before-define` eslint rule has been added to catch these
cases going forward. The rule is disabled for function declarations for
the moment, because those are always hoisted. We could disable that too
if we want to, but it's purely stylistic and would require a lot more
changes.
* eslint: Check for unused function arguments
* eslint: Ignore unused '_' in argument list
Also allow any number of '_' e.g., '__' or '___' which is to be used sparingly
* Remove and rename unused arguments
