Add background check to test.

Author: mmalerba

src/lib/slider/slider.spec.ts

@@ -329,6 +329,9 @@ describe('MdSlider', () => {
       expect(trackFillDimensions.width).toBe(sliderDimensions.width * 6 / 8);
       expect(tickContainerDimensions.width)
           .toBe(sliderDimensions.width - sliderDimensions.width * 6 / 8);
+      expect(tickContainerElement.style.background)
+          .toEqual(`repeating-linear-gradient(to right, black, black 2px, transparent 2px, ` +
+              `transparent ${sliderDimensions.width * 6 / 8 - 1}px)`);
     });
 
     it('should adjust thumb and ticks when max changes', () => {
@@ -344,6 +347,9 @@ describe('MdSlider', () => {
       expect(trackFillDimensions.width).toBe(sliderDimensions.width * 6 / 12);
       expect(tickContainerDimensions.width)
           .toBe(sliderDimensions.width - sliderDimensions.width * 6 / 12);
+      expect(tickContainerElement.style.background)
+          .toEqual(`repeating-linear-gradient(to right, black, black 2px, transparent 2px, ` +
+              `transparent ${sliderDimensions.width * 6 / 12 - 1}px)`);
     });
   });