fix(select): multiple change events emitted when changing options of a closed select (#7232)

Prevents two change events being fired the same change when selecting an option via the arrow keys on a closed select.

Fixes #7227.

Author: crisbeto

src/lib/select/select.spec.ts

@@ -2336,6 +2336,14 @@ describe('MatSelect', () => {
 
       expect(fixture.componentInstance.changeListener).toHaveBeenCalledTimes(1);
     });
+
+    it('should only emit one event when pressing the arrow keys on a closed select', () => {
+      const select = fixture.debugElement.query(By.css('md-select')).nativeElement;
+      dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+      expect(fixture.componentInstance.changeListener).toHaveBeenCalledTimes(1);
+    });
+
   });
 
   describe('floatPlaceholder option', () => {

src/lib/select/select.ts

@@ -1135,7 +1135,6 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
       if (currentActiveItem !== prevActiveItem) {
         this._clearSelection();
         this._setSelectionByValue(currentActiveItem.value, true);
-        this._propagateChanges();
       }
     }
   }