src/vs/platform/files/common/inMemoryFilesystemProvider.ts/InMemoryFileSystemProvider/_lookupAsFile
class InMemoryFileSystemProvider:	private _lookupAsFile(uri: URI, silent: boolean): File {
		let entry = this._lookup(uri, silent);
		if (entry instanceof File) {
			return entry;
		}
		throw new FileSystemProviderError('file is a directory', FileSystemProviderErrorCode.FileIsADirectory);
	}