[no-unnecessary-type-assertion] false positive on uninitialized variables

Repro

{
  "rules": {
    "@typescript-eslint/no-unnecessary-type-assertion": "error"
  }
}

function test() {
    let resolve: () => void
    new Promise(resolve0 => {
        resolve = resolve0
    })
    return resolve!
}

Expected Result

No errors because it's a syntax error if I removed the !.

Actual Result

6:12  error  This assertion is unnecessary since it does not change the type of the expression  @typescript-eslint/no-unnecessary-type-assertion

Additional Info

Versions

package	version
`@typescript-eslint/eslint-plugin`	`1.7.0`
`@typescript-eslint/parser`	`1.7.0`
`TypeScript`	`3.4.3`
`ESLint`	`5.16.0`
`node`	`10.15.3`
`npm`	`6.9.0`

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