Created using remix-ide: Realtime Ethereum Contract Compiler and Runtime. Load this file by pasting this gists URL or ID at https://remix.ethereum.org/#version=soljson-v0.5.1+commit.c8a2cb62.js&optimize=true&gist=

Animal.sol

pragma solidity 0.5.1;

import './Ownable.sol';

contract AnimalContract is Ownable{
    
    event animalAdded(address owner, string animalName);
    
    enum AnimalType {DOG, CAT}
    
    struct Animal {
        string name;
        uint age;
        AnimalType animalType;
    }
    
    mapping (address => Animal[]) ownerToAnimals;
    
    function _addAnimal(string memory _name, uint _age, AnimalType _animalType) internal onlyOwner returns (uint) {  
        emit animalAdded(msg.sender, _name);
        return ownerToAnimals[msg.sender].push(Animal(_name, _age, _animalType)) - 1;
    }
    
    function getAnimal(uint _id) public view returns (string memory){
        return ownerToAnimals[msg.sender][_id].name;
    }
}

ballot.sol

pragma solidity >=0.4.22 <0.6.0;
contract Ballot {

    struct Voter {
        uint weight;
        bool voted;
        uint8 vote;
        address delegate;
    }
    struct Proposal {
        uint voteCount;
    }

    address chairperson;
    mapping(address => Voter) voters;
    Proposal[] proposals;

    /// Create a new ballot with $(_numProposals) different proposals.
    constructor(uint8 _numProposals) public {
        chairperson = msg.sender;
        voters[chairperson].weight = 1;
        proposals.length = _numProposals;
    }

    /// Give $(toVoter) the right to vote on this ballot.
    /// May only be called by $(chairperson).
    function giveRightToVote(address toVoter) public {
        if (msg.sender != chairperson || voters[toVoter].voted) return;
        voters[toVoter].weight = 1;
    }

    /// Delegate your vote to the voter $(to).
    function delegate(address to) public {
        Voter storage sender = voters[msg.sender]; // assigns reference
        if (sender.voted) return;
        while (voters[to].delegate != address(0) && voters[to].delegate != msg.sender)
            to = voters[to].delegate;
        if (to == msg.sender) return;
        sender.voted = true;
        sender.delegate = to;
        Voter storage delegateTo = voters[to];
        if (delegateTo.voted)
            proposals[delegateTo.vote].voteCount += sender.weight;
        else
            delegateTo.weight += sender.weight;
    }

    /// Give a single vote to proposal $(toProposal).
    function vote(uint8 toProposal) public {
        Voter storage sender = voters[msg.sender];
        if (sender.voted || toProposal >= proposals.length) return;
        sender.voted = true;
        sender.vote = toProposal;
        proposals[toProposal].voteCount += sender.weight;
    }

    function winningProposal() public view returns (uint8 _winningProposal) {
        uint256 winningVoteCount = 0;
        for (uint8 prop = 0; prop < proposals.length; prop++)
            if (proposals[prop].voteCount > winningVoteCount) {
                winningVoteCount = proposals[prop].voteCount;
                _winningProposal = prop;
            }
    }
}

ballot_test.sol

import "remix_tests.sol"; // this import is automatically injected by Remix.
import "./ballot.sol";

contract test3 {
   
    Ballot ballotToTest;
    function beforeAll () public {
       ballotToTest = new Ballot(2);
    }
    
    function checkWinningProposal () public {
        ballotToTest.vote(1);
        Assert.equal(ballotToTest.winningProposal(), uint(1), "1 should be the winning proposal");
    }
    
    function checkWinninProposalWithReturnValue () public view returns (bool) {
        return ballotToTest.winningProposal() == 1;
    }
}

Dog.sol

pragma solidity 0.5.1;

import './Animal.sol';

contract DogContract is AnimalContract {
    
    modifier costs(uint amount){
        require (msg.value >= amount);
        _;
        if(msg.value > amount){
            msg.sender.transfer(msg.value - amount);
        }
    }
    
    function addDog(string memory _name, uint _age) public payable costs(500) returns (uint){
        return _addAnimal(_name, _age, AnimalType.DOG);
    }
    
    function getBalance() public view returns (uint) {
        return address(this).balance;
    }

}

DogInterface.sol

pragma solidity 0.5.1;

contract DogContract{

    function addDog(string memory _name, uint _age) public payable returns (uint);

    function getBalance() public view returns (uint);
}

contract ExternalContract {

    DogContract dc = DogContract(0x61Ba7517F22c0122aFd02b597f9DeA0318cae34C);

    function addExternalDog(string memory _name, uint _age) public payable returns (uint){
        return dc.addDog.value(msg.value)(_name, _age);
    }
    
    function getExternalBalance() public view returns (uint){
        return dc.getBalance();
    }
}

IntegetUnderOverFlow.sol

pragma solidity 0.5.1;

contract C {

    // (2**256 - 1) + 1 = 0
    function overflow() public pure returns (uint256 _overflow) {
        uint256 max = 2**256 -1;
        return max + 1;
    }

    // 0 - 1 = 2**256 - 1
    function underflow() public pure returns (uint256 _underflow) {
        uint256 min = 0;
        return min - 1;
    }
}

Ownable.sol

pragma solidity 0.5.1;

contract Ownable{
    
    address public owner;
    
    modifier onlyOwner(){
        require (msg.sender == owner);
        _;
    }
    
    constructor() public {
        owner = msg.sender;
    }
    
    function transferOwnership(address newOwner) public onlyOwner {
        // require(newOwner != address(0));
        owner = newOwner;
    }

}

SafeMathAdded.sol

pragma solidity 0.5.1;

library SafeMath {

  /**
  * @dev Multiplies two numbers, throws on overflow.
  */
  function mul(uint256 a, uint256 b) internal pure returns (uint256 c) {
    // Gas optimization: this is cheaper than asserting 'a' not being zero, but the
    // benefit is lost if 'b' is also tested.
    // See: https://github.com/OpenZeppelin/openzeppelin-solidity/pull/522
    if (a == 0) {
      return 0;
    }

    c = a * b;
    assert(c / a == b);
    return c;
  }

  /**
  * @dev Integer division of two numbers, truncating the quotient.
  */
  function div(uint256 a, uint256 b) internal pure returns (uint256) {
    // assert(b > 0); // Solidity automatically throws when dividing by 0
    // uint256 c = a / b;
    // assert(a == b * c + a % b); // There is no case in which this doesn't hold
    return a / b;
  }

  /**
  * @dev Subtracts two numbers, throws on overflow (i.e. if subtrahend is greater than minuend).
  */
  function sub(uint256 a, uint256 b) internal pure returns (uint256) {
    assert(b <= a);
    return a - b;
  }

  /**
  * @dev Adds two numbers, throws on overflow.
  */
  function add(uint256 a, uint256 b) internal pure returns (uint256 c) {
    c = a + b;
    assert(c >= a);
    return c;
  }
}


contract C {
    using SafeMath for uint256;

    // (2**256 - 1) + 1 = 0
    function overflow() public pure returns (uint256 _overflow) {
        uint256 max = 2**256 -1;
        return max.add(1);
    }

    // 0 - 1 = 2**256 - 1
    function underflow() public pure returns (uint256 _underflow) {
        uint256 min = 0;
        return min.sub(1);
    }
}