function assertLengthChain () {
        flag(this, 'doLength', true);
      }
### .length(value)

Asserts that the target's `length` property has
the expected value.

    expect([ 1, 2, 3]).to.have.length(3);
    expect('foobar').to.have.length(6);

Can also be used as a chain precursor to a value
comparison for the length property.

    expect('foo').to.have.length.above(2);
    expect([ 1, 2, 3 ]).to.have.length.above(2);
    expect('foo').to.have.length.below(4);
    expect([ 1, 2, 3 ]).to.have.length.below(4);
    expect('foo').to.have.length.within(2,4);
    expect([ 1, 2, 3 ]).to.have.length.within(2,4);

@name length
@alias lengthOf
@param {Number} length
@param {String} message _optional_
@api public