fix(stepper): selected is always undefined (#7213)

Author: NothingEverHappens

src/cdk/stepper/stepper.ts

@@ -150,7 +150,7 @@ export class CdkStepper {
 
   /** The step that is selected. */
   @Input()
-  get selected() { return this._steps[this.selectedIndex]; }
+  get selected() { return this._steps.toArray()[this.selectedIndex]; }
   set selected(step: CdkStep) {
     this.selectedIndex = this._steps.toArray().indexOf(step);
   }

src/lib/stepper/stepper.spec.ts

@@ -10,7 +10,6 @@ import {MatStepperModule} from './index';
 import {MatHorizontalStepper, MatStepper, MatVerticalStepper} from './stepper';
 import {MatStepperNext, MatStepperPrevious} from './stepper-button';
 
-
 const VALID_REGEX = /valid/;
 
 describe('MatHorizontalStepper', () => {
@@ -307,20 +306,23 @@ function assertSelectionChangeOnHeaderClick(fixture: ComponentFixture<any>,
   let stepperComponent = fixture.debugElement.query(By.directive(MatStepper)).componentInstance;
 
   expect(stepperComponent.selectedIndex).toBe(0);
+  expect(stepperComponent.selected instanceof MdStep).toBe(true);
 
   // select the second step
   let stepHeaderEl = stepHeaders[1].nativeElement;
   stepHeaderEl.click();
   fixture.detectChanges();
 
   expect(stepperComponent.selectedIndex).toBe(1);
+  expect(stepperComponent.selected instanceof MdStep).toBe(true);
 
   // select the third step
   stepHeaderEl = stepHeaders[2].nativeElement;
   stepHeaderEl.click();
   fixture.detectChanges();
 
   expect(stepperComponent.selectedIndex).toBe(2);
+  expect(stepperComponent.selected instanceof MdStep).toBe(true);
 }
 
 /** Asserts that 'aria-expanded' attribute is correct for expanded content of step. */