fix(datepicker): calendar toggle submitting parent form

Prevents the toggle for the datepicker's calendar from submitting its parent form.

Fixes #4530.

Author: crisbeto

src/lib/datepicker/datepicker-toggle.ts

@@ -9,6 +9,7 @@ import {MdDatepickerIntl} from './datepicker-intl';
   template: '',
   styleUrls: ['datepicker-toggle.css'],
   host: {
+    'type': 'button', // prevents accidental form submissions
     '[class.mat-datepicker-toggle]': 'true',
     '[attr.aria-label]': '_intl.openCalendarLabel',
     '(click)': '_open($event)',

src/lib/datepicker/datepicker.spec.ts

@@ -374,15 +374,20 @@ describe('MdDatepicker', () => {
         fixture.detectChanges();
       }));
 
-      it('should open calendar when toggle clicked', async(() => {
+      it('should open calendar when toggle clicked', () => {
         expect(document.querySelector('md-dialog-container')).toBeNull();
 
         let toggle = fixture.debugElement.query(By.css('button'));
         dispatchMouseEvent(toggle.nativeElement, 'click');
         fixture.detectChanges();
 
         expect(document.querySelector('md-dialog-container')).not.toBeNull();
-      }));
+      });
+
+      it('should set the `button` type on the trigger to prevent form submissions', () => {
+        let toggle = fixture.debugElement.query(By.css('button')).nativeElement;
+        expect(toggle.getAttribute('type')).toBe('button');
+      });
     });
 
     describe('datepicker inside input-container', () => {