fix type issue

Author: mmalerba

src/cdk-experimental/scrolling/virtual-scroll-viewport.scss

@@ -13,15 +13,15 @@
     padding: {
       #{$start}: 0;
       #{$end}: 0
-    };
+    }
     margin: {
       #{$start}: 0;
       #{$end}: 0
-    };
+    }
     border: {
       #{$start}-width: 0;
       #{$end}-width: 0
-    };
+    }
     outline: none;
   }
 }

src/cdk-experimental/scrolling/virtual-scroll-viewport.spec.ts

@@ -27,7 +27,7 @@ describe('CdkVirtualScrollViewport', () => {
       finishInit(fixture);
 
       const contentWrapper =
-          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper');
+          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper')!;
       expect(contentWrapper.children.length)
           .toBe(4, 'should render 4 50px items to fill 200px space');
     }));
@@ -507,7 +507,7 @@ describe('CdkVirtualScrollViewport', () => {
       finishInit(fixture);
 
       const contentWrapper =
-          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper');
+          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper')!;
       expect(contentWrapper.children.length)
           .toBe(4, 'should render 4 50px items to fill 200px space');
     }));
@@ -517,7 +517,7 @@ describe('CdkVirtualScrollViewport', () => {
       finishInit(fixture);
 
       const contentWrapper =
-          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper');
+          viewport.elementRef.nativeElement.querySelector('.cdk-virtual-scroll-content-wrapper')!;
       expect(contentWrapper.children.length).toBe(4,
           'should render 4 items to fill 200px space based on 50px estimate from first item');
     }));