The error message "Member "equal" is not available in the library assistant" appears

Error: Member "equal" is not available in type(library Assert) outside of storage.
        Assert.equal(token.symbol(), "$", "The symbol of the token should be $");
        ^----------^
Compiliation failed. See above.

pragma solidity ^0.4.8; 
contract Token { 
    /* /* The amount of tokens a person will get for 1 ETH */ 
    uint256 public exchangeRate; 

    /* /* The name of the token */ 
    string public name; 

    /* /* The address which controls the token */ 
    address public owner; 

    /* /* The symbol of the token */ 
    string public symbol; 

    /* /* The balances of all registered addresses */ 
    mapping (address => uint256) balances; 

    /* /* Token constructor */ 
    function Token(uint256 _exchangeRate, string _name, string _symbol) { 
        exchangeRate = _exchangeRate; 
        name = _name; 
        owner = msg.sender; 
        symbol = _symbol; 
    }

    function getBalance(address account) returns (uint256 balance) {
          return balances[account]; 
    }
}

pragma solidity ^0.4.8;

// // Framework libraries
import "truffle/Assert.sol";
import "truffle/DeployedAddresses.sol";

// // Custom libraries and contracts
import "../contracts/Token.sol";

contract TestToken {
    function testExchangeRate() {
        Token token = new Token(500, "Dollar", "$");

        uint256 expected = 500;

        Assert.equal(token.exchangeRate(), expected, "The exchange rate should be 500 tokens for 1 ETH");
    }

    function testSymbol() {
        Token token = new Token(500, "Dollar", "$");

        Assert.equal(token.symbol(), "$", "The symbol of the token should be $");
    }
}