build: bump to tslint 5.8.0 and fix failures (#7943)

Updates to `tslint@5.8.0` and fixes a few previously-uncaught failures.

src/lib/core/option/option.ts

@@ -110,7 +110,7 @@ export class MatOption {
     private _element: ElementRef,
     private _changeDetectorRef: ChangeDetectorRef,
     @Optional() @Inject(MAT_OPTION_PARENT_COMPONENT) private _parent: MatOptionParentComponent,
-    @Optional() public readonly group: MatOptgroup) {}
+    @Optional() readonly group: MatOptgroup) {}
 
   /**
    * Whether or not the option is currently active and ready to be selected.

src/lib/dialog/dialog-ref.ts

@@ -44,7 +44,7 @@ export class MatDialogRef<T> {
   constructor(
     private _overlayRef: OverlayRef,
     private _containerInstance: MatDialogContainer,
-    public readonly id: string = `mat-dialog-${uniqueId++}`) {
+    readonly id: string = `mat-dialog-${uniqueId++}`) {
 
     // Emit when opening animation completes
     RxChain.from(_containerInstance._animationStateChanged)