src/vs/workbench/api/common/extHostCustomEditors.ts/CustomDocumentStore/add
class CustomDocumentStore:	public add(viewType: string, document: vscode.CustomDocument, storagePath: URI | undefined): CustomDocumentStoreEntry {
		const key = this.key(viewType, document.uri);
		if (this._documents.has(key)) {
			throw new Error(`Document already exists for viewType:${viewType} resource:${document.uri}`);
		}
		const entry = new CustomDocumentStoreEntry(document, storagePath);
		this._documents.set(key, entry);
		return entry;
	}