fix(select): handle keyboard events from inside panel

After the switch to using `aria-describedby` for managing the highlights in `mat-select`, all of the keyboard handling was moved to the select trigger, however this doesn't account for the cases where focus makes it inside the panel (e.g. when toggling options in `multiple` mode).

Author: crisbeto

src/lib/select/select.html

@@ -37,7 +37,8 @@
     (@transformPanel.done)="_onPanelDone()"
     [style.transformOrigin]="_transformOrigin"
     [class.mat-select-panel-done-animating]="_panelDoneAnimating"
-    [style.font-size.px]="_triggerFontSize">
+    [style.font-size.px]="_triggerFontSize"
+    (keydown)="_handleKeydown($event)">
 
     <div
       class="mat-select-content"

src/lib/select/select.spec.ts

@@ -759,8 +759,6 @@ describe('MatSelect', () => {
         expect(pane.style.minWidth).toBe('200px');
       }));
 
-
-
       it('should not attempt to open a select that does not have any options', fakeAsync(() => {
         fixture.componentInstance.foods = [];
         fixture.detectChanges();
@@ -771,8 +769,6 @@ describe('MatSelect', () => {
         expect(fixture.componentInstance.select.panelOpen).toBe(false);
       }));
 
-
-
       it('should close the panel when tabbing out', fakeAsync(() => {
         trigger.click();
         fixture.detectChanges();
@@ -787,6 +783,21 @@ describe('MatSelect', () => {
         expect(fixture.componentInstance.select.panelOpen).toBe(false);
       }));
 
+      it('should close when tabbing out from inside the panel', fakeAsync(() => {
+        trigger.click();
+        fixture.detectChanges();
+        flush();
+
+        expect(fixture.componentInstance.select.panelOpen).toBe(true);
+
+        const panel = overlayContainerElement.querySelector('.mat-select-panel')!;
+        dispatchKeyboardEvent(panel, 'keydown', TAB);
+        fixture.detectChanges();
+        flush();
+
+        expect(fixture.componentInstance.select.panelOpen).toBe(false);
+      }));
+
       it('should focus the first option when pressing HOME', fakeAsync(() => {
         fixture.componentInstance.control.setValue('pizza-1');
         fixture.detectChanges();