extensions/markdown-language-features/src/util/workspaceCache.ts/MdDocumentInfoCache/loadDocument
class MdDocumentInfoCache:	private loadDocument(resource: vscode.Uri): Promise<ITextDocument | undefined> {
		const existing = this._loadingDocuments.get(resource);
		if (existing) {
			return existing;
		}

		const p = this.workspace.getOrLoadMarkdownDocument(resource);
		this._loadingDocuments.set(resource, p);
		p.finally(() => {
			this._loadingDocuments.delete(resource);
		});
		return p;
	}