fix(tooltip): error on trigger escape presses while closed (#7028)

Fixes an error being thrown when pressing escape on a tooltip trigger that isn't showing a tooltip.

Fixes #7009.

src/lib/tooltip/tooltip.spec.ts

@@ -19,7 +19,8 @@ import {NoopAnimationsModule} from '@angular/platform-browser/animations';
 import {Direction, Directionality} from '@angular/cdk/bidi';
 import {OverlayContainer, OverlayModule, Scrollable} from '@angular/cdk/overlay';
 import {Platform} from '@angular/cdk/platform';
-import {dispatchFakeEvent} from '@angular/cdk/testing';
+import {dispatchFakeEvent, dispatchKeyboardEvent} from '@angular/cdk/testing';
+import {ESCAPE} from '@angular/cdk/keycodes';
 import {
   MdTooltip,
   MdTooltipModule,
@@ -469,6 +470,13 @@ describe('MdTooltip', () => {
       expect(overlayContainerElement.textContent).toContain(initialTooltipMessage);
     }));
 
+    it('should not throw when pressing ESCAPE', fakeAsync(() => {
+      expect(() => {
+        dispatchKeyboardEvent(buttonElement, 'keydown', ESCAPE);
+        fixture.detectChanges();
+      }).not.toThrow();
+    }));
+
   });
 
   describe('scrollable usage', () => {

src/lib/tooltip/tooltip.ts

@@ -270,7 +270,7 @@ export class MdTooltip implements OnDestroy {
 
   /** Handles the keydown events on the host element. */
   _handleKeydown(e: KeyboardEvent) {
-    if (this._tooltipInstance!.isVisible() && e.keyCode === ESCAPE) {
+    if (this._isTooltipVisible() && e.keyCode === ESCAPE) {
       e.stopPropagation();
       this.hide(0);
     }