fix(autocomplete): don't block default arrow keys when using modifiers

Currently we hijack all up/down arrow key events, however this interferes with keyboard shortcuts such as shift + up arrow for marking all of the text. These changes stop intercepting the arrow keys, if they're used with a modifier.

These changes also fix an issue where all the mocked out key events had the `metaKey` set to `true` on some browsers.

src/material/autocomplete/autocomplete-trigger.ts

@@ -412,15 +412,16 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, AfterViewIn
       event.preventDefault();
     } else if (this.autocomplete) {
       const prevActiveItem = this.autocomplete._keyManager.activeItem;
-      const isArrowKey = keyCode === UP_ARROW || keyCode === DOWN_ARROW;
+      const isVerticalArrowKey = keyCode === UP_ARROW || keyCode === DOWN_ARROW;
+      const hasModifier = event.ctrlKey || event.altKey || event.metaKey || event.shiftKey;
 
-      if (this.panelOpen || keyCode === TAB) {
+      if ((isVerticalArrowKey && !hasModifier) || keyCode === TAB) {
         this.autocomplete._keyManager.onKeydown(event);
-      } else if (isArrowKey && this._canOpen()) {
+      } else if (isVerticalArrowKey && this._canOpen()) {
         this.openPanel();
       }
 
-      if (isArrowKey || this.autocomplete._keyManager.activeItem !== prevActiveItem) {
+      if (isVerticalArrowKey || this.autocomplete._keyManager.activeItem !== prevActiveItem) {
         this._scrollToOption();
       }
     }

src/material/autocomplete/autocomplete.spec.ts

@@ -1250,6 +1250,18 @@ describe('MatAutocomplete', () => {
       expect(!!trigger.activeOption).toBe(false, 'Expected no active options.');
     }));
 
+    it('should not prevent the default action when a modifier key is pressed', () => {
+      ['metaKey', 'ctrlKey', 'altKey', 'shiftKey'].forEach(name => {
+        const event = createKeyboardEvent('keydown', DOWN_ARROW);
+        Object.defineProperty(event, name, {get: () => true});
+
+        fixture.componentInstance.trigger._handleKeydown(event);
+        fixture.detectChanges();
+
+        expect(event.defaultPrevented).toBe(false, `Expected autocompete not to block ${name} key`);
+      });
+    });
+
   });
 
   describe('option groups', () => {