old_contents: /**
   * The MIT License (MIT)
   *
   * Copyright (c) 2015 Mickael Jeanroy
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in all
   * copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
   * SOFTWARE.
   */
  
  describe('Stack', function() {
    it('should create a new stack', function() {
      var stack = new Stack();
      expect(stack.root).toBeNull();
    });
  
    it('should push value onto the stack', function() {
      var stack = new Stack();
  
      stack.push('foo');
  
      expect(stack.root).toEqual(jasmine.objectContaining({
        value: 'foo',
        next: null
      }));
  
      stack.push('bar');
  
      expect(stack.root).toEqual(jasmine.objectContaining({
        value: 'bar',
        next: jasmine.objectContaining({
          value: 'foo',
          next: null
        })
      }));
    });
  
    it('should peek value from the stack', function() {
      var stack = new Stack();
  
      stack.push('foo');
      expect(stack.peek()).toBe('foo');
      expect(stack.peek()).toBe('foo');
  
      stack.push('bar');
      expect(stack.peek()).toBe('bar');
      expect(stack.peek()).toBe('bar');
    });
  
    it('should pop value from the stack', function() {
      var stack = new Stack();
  
      stack.push('foo');
      stack.push('bar');
  
      expect(stack.pop()).toBe('bar');
      expect(stack.pop()).toBe('foo');
      expect(stack.pop()).toBeUndefined();
    });
  
    it('should check if stack is empty', function() {
      var stack = new Stack();
      expect(stack.isEmpty()).toBeTrue();
  
      stack.push('bar');
      expect(stack.isEmpty()).toBeFalse();
  
      stack.pop();
      expect(stack.isEmpty()).toBeTrue();
    });
  });
  
new_contents: /**
   * The MIT License (MIT)
   *
   * Copyright (c) 2015 Mickael Jeanroy
   *
   * Permission is hereby granted, free of charge, to any person obtaining a copy
   * of this software and associated documentation files (the "Software"), to deal
   * in the Software without restriction, including without limitation the rights
   * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   * copies of the Software, and to permit persons to whom the Software is
   * furnished to do so, subject to the following conditions:
   *
   * The above copyright notice and this permission notice shall be included in all
   * copies or substantial portions of the Software.
   *
   * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
   * SOFTWARE.
   */
  
  describe('Stack', () => {
    it('should create a new stack', () => {
      const stack = new Stack();
      expect(stack.root).toBeNull();
    });
  
    it('should push value onto the stack', () => {
      const stack = new Stack();
  
      stack.push('foo');
  
      expect(stack.root).toEqual(jasmine.objectContaining({
        value: 'foo',
        next: null
      }));
  
      stack.push('bar');
  
      expect(stack.root).toEqual(jasmine.objectContaining({
        value: 'bar',
        next: jasmine.objectContaining({
          value: 'foo',
          next: null
        })
      }));
    });
  
    it('should peek value from the stack', () => {
      const stack = new Stack();
  
      stack.push('foo');
      expect(stack.peek()).toBe('foo');
      expect(stack.peek()).toBe('foo');
  
      stack.push('bar');
      expect(stack.peek()).toBe('bar');
      expect(stack.peek()).toBe('bar');
    });
  
    it('should pop value from the stack', () => {
      const stack = new Stack();
  
      stack.push('foo');
      stack.push('bar');
  
      expect(stack.pop()).toBe('bar');
      expect(stack.pop()).toBe('foo');
      expect(stack.pop()).toBeUndefined();
    });
  
    it('should check if stack is empty', () => {
      const stack = new Stack();
      expect(stack.isEmpty()).toBeTrue();
  
      stack.push('bar');
      expect(stack.isEmpty()).toBeFalse();
  
      stack.pop();
      expect(stack.isEmpty()).toBeTrue();
    });
  });
  
subject: migrate stack-spec to es6