Flow generics trigger no-shadow rule

Declarations like:
```
export function foo(bar: T) {
return bar
}
```

are marked by the linter as `'T' is already declared in the upper scope. (no-shadow)`

Versions:
```
"babel-eslint": "8.0.0",
"eslint": "4.7.2",
"eslint-config-airbnb": "15.1.0",
"eslint-config-prettier": "2.6.0",
"eslint-plugin-flowtype": "2.36.0",
"eslint-plugin-import": "2.7.0",
"eslint-plugin-jsx-a11y": "6.0.2",
"eslint-plugin-prettier": "2.3.1",
"eslint-plugin-react": "7.4.0",
"flow-bin": "0.55.0",
```

Start by reproducing the reported no-shadow warning with the Flow generic declaration and the listed ESLint, babel-eslint, and Flow versions. Trace how the no-shadow rule handles the generic type parameter; done means valid declarations like foo no longer produce the warning while genuine shadowing remains reported.

Written by the indexing model from the issue text.