Broken syntax highlighting when combine overloading and export default

[HitalloExiled]

• VSCode Version: 1.26.1
• OS Version: Windows 10

Steps to Reproduce:

//index.ts
export default function foo(): void;
export default function foo(bar: string): void;
export default function foo(bar?: string): void
{ }

Does this issue occur when all extensions are disabled?: Yes

[mjbvz]

This issue was moved to microsoft/TypeScript-TmLanguage#663