Typescript syntax highlighting fails for arrow functions with multiline arguments

Visual Studio Code 1.18.1
Typescript 2.6.2

See example below:

```ts
export const foo = (firstVariable: string, secondVariable: number): string => {
    return "foo"
}

export const bar = (
    firstVariable: string,
    secondVariable: number,
    thirdVariable: string
): string => {
    return "baz"
}
```

