fix(spinner): set initial value for spinner to 0. (#8139)

src/lib/progress-spinner/progress-spinner.spec.ts

@@ -37,12 +37,12 @@ describe('MatProgressSpinner', () => {
     expect(progressElement.componentInstance.mode).toBe('indeterminate');
   });
 
-  it('should define a default value of undefined for the value attribute', () => {
+  it('should define a default value of zero for the value attribute', () => {
     let fixture = TestBed.createComponent(BasicProgressSpinner);
     fixture.detectChanges();
 
     let progressElement = fixture.debugElement.query(By.css('mat-progress-spinner'));
-    expect(progressElement.componentInstance.value).toBeUndefined();
+    expect(progressElement.componentInstance.value).toBe(0);
   });
 
   it('should set the value to 0 when the mode is set to indeterminate', () => {

src/lib/progress-spinner/progress-spinner.ts

@@ -83,7 +83,7 @@ const INDETERMINATE_ANIMATION_TEMPLATE = `
 export class MatProgressSpinner extends _MatProgressSpinnerMixinBase implements CanColor,
   OnChanges {
 
-  private _value: number;
+  private _value: number = 0;
   private readonly _baseSize = 100;
   private readonly _baseStrokeWidth = 10;
   private _fallbackAnimation = false;
@@ -119,7 +119,7 @@ export class MatProgressSpinner extends _MatProgressSpinnerMixinBase implements
 
   /** Value of the progress circle. */
   @Input()
-  get value() {
+  get value(): number {
     return this.mode === 'determinate' ? this._value : 0;
   }
   set value(newValue: number) {