Lazy vars shared examples

spec.js

sharedExamplesFor('Collection', function() {
  it('is empty by default', function() {
    expect($subject).to.be.empty;
  });
  
  it('increments "length" property when adds a new item', function() {
    $subject.add(1, 2, 3);
    expect($subject).to.have.length(3);
  });
  
  it('can remove item', function() {
    $subject.add(1, 2);
    $subject.remove(2);
    expect($subject).not.to.contain(2);
  });
});