In order to get current block height you need to call method getCurrentBlock() it will return you a struct with following signature:
struct Block {
let id: [UInt8; 32]
let height: UInt64
let timestamp: UFix64
}
Current version of Playground have this method implemented, though it returns you a struct with two fields number (which should represent block height) and hash. We would advice to mock this method for now and then replace it with actual call of native method. Sorry for inconvenience 
