src/vs/workbench/api/common/extHostLanguages.ts/ExtHostLanguages/changeLanguage
class ExtHostLanguages:	async changeLanguage(uri: vscode.Uri, languageId: string): Promise<vscode.TextDocument> {
		await this._proxy.$changeLanguage(uri, languageId);
		const data = this._documents.getDocumentData(uri);
		if (!data) {
			throw new Error(`document '${uri.toString}' NOT found`);
		}
		return data.document;
	}