Fixed checking of abstract functions.
Fixed checking of abstract functions.

Fixes #2264

ruchevits/solidity,LianaHus/solidity,ruchevits/solidity,chriseth/solidity-doc-test,douglas-larocca/solidity,ruchevits/solidity,arkpar/solidity,vaporry/solidity,gluk256/solidity,shahankhatch/solidity,ruchevits/solidity,debris/solidity,ethers/solidity,chriseth/solidity