src/vs/workbench/api/common/extHostTypes.ts/RelativePattern/constructor
class RelativePattern:	constructor(base: vscode.WorkspaceFolder | URI | string, pattern: string) {
		if (typeof base !== 'string') {
			if (!base || !URI.isUri(base) && !URI.isUri(base.uri)) {
				throw illegalArgument('base');
			}
		}

		if (typeof pattern !== 'string') {
			throw illegalArgument('pattern');
		}

		if (typeof base === 'string') {
			this.baseUri = URI.file(base);
		} else if (URI.isUri(base)) {
			this.baseUri = base;
		} else {
			this.baseUri = base.uri;
		}

		this.pattern = pattern;
	}