Preserve type aliases on implement interface

<!-- 🚨 STOP 🚨 𝗦𝗧𝗢𝗣 🚨 𝑺𝑻𝑶𝑷 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> From https://github.com/microsoft/vscode/issues/90061 <!-- Please try to reproduce the issue with the latest published version. It may have already been fixed. For npm: `typescript@next` This is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly --> **TypeScript Version:** 3.8.1-rc <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** - quick fix - implement interface **Repo** ```ts export type UrlString = string; export type UUID = string; export interface I_IdToUrlMapper { mapIdToUrl(id: UUID): UrlString mapUrlToId(url: UrlString): UUID } export class IdToUrlMapper implements I_IdToUrlMapper { } ``` Run implement interface on `IdToUrlMapper ` **Expected behavior:** Type aliases are preserved ```ts export class IdToUrlMapper implements I_IdToUrlMapper { mapIdToUrl(id: UUID): UrlString { throw new Error("Method not implemented."); } mapUrlToId(url: UrlString): UUID { throw new Error("Method not implemented."); } } ``` **Actual behavior:** Both methods use `string`: ```ts export class IdToUrlMapper implements I_IdToUrlMapper { mapIdToUrl(id: string): string { throw new Error("Method not implemented."); } mapUrlToId(url: string): string { throw new Error("Method not implemented."); } } ``` **Related Issues** - https://github.com/microsoft/TypeScript/issues/34778