perf: memory leak when subscribing to zone events

Fixes a few memory leaks that were caused by subscribing to `NgZone.onMicrotaskEmpty` or `NgZone.onStable`. Because the two streams are `EventEmitters`, the subscription doesn't get GC-ed correctly. These changes switch to converting the emitters to observables before subscribing to them.

Fxes angular#6905.

src/cdk/a11y/focus-trap.ts

@@ -265,7 +265,7 @@ export class FocusTrap {
     if (this._ngZone.isStable) {
       fn();
     } else {
-      first.call(this._ngZone.onStable).subscribe(fn);
+      first.call(this._ngZone.onStable.asObservable()).subscribe(fn);
     }
   }
 }

src/lib/autocomplete/autocomplete-trigger.ts

@@ -372,7 +372,7 @@ export class MdAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
    * stream every time the option list changes.
    */
   private _subscribeToClosingActions(): Subscription {
-    const firstStable = first.call(this._zone.onStable);
+    const firstStable = first.call(this._zone.onStable.asObservable());
     const optionChanges = map.call(this.autocomplete.options.changes, () =>
       this._positionStrategy.recalculateLastPosition());
 

src/lib/datepicker/calendar.ts

@@ -218,9 +218,11 @@ export class MdCalendar<D> implements AfterContentInit, OnDestroy {
 
   /** Focuses the active cell after the microtask queue is empty. */
   _focusActiveCell() {
-    this._ngZone.runOutsideAngular(() => first.call(this._ngZone.onStable).subscribe(() => {
-      this._elementRef.nativeElement.querySelector('.mat-calendar-body-active').focus();
-    }));
+    this._ngZone.runOutsideAngular(() => {
+      first.call(this._ngZone.onStable.asObservable()).subscribe(() => {
+        this._elementRef.nativeElement.querySelector('.mat-calendar-body-active').focus();
+      });
+    });
   }
 
   /** Whether the two dates represent the same view in the current view mode (month or year). */

src/lib/datepicker/datepicker.ts

@@ -313,7 +313,9 @@ export class MdDatepicker<D> implements OnDestroy {
       componentRef.instance.datepicker = this;
 
       // Update the position once the calendar has rendered.
-      first.call(this._ngZone.onStable).subscribe(() => this._popupRef.updatePosition());
+      first.call(this._ngZone.onStable.asObservable()).subscribe(() => {
+        this._popupRef.updatePosition();
+      });
     }
 
     this._popupRef.backdropClick().subscribe(() => this.close());

src/lib/sidenav/drawer.ts

@@ -401,8 +401,11 @@ export class MdDrawerContainer implements AfterContentInit {
     }
     // NOTE: We need to wait for the microtask queue to be empty before validating,
     // since both drawers may be swapping positions at the same time.
-    takeUntil.call(drawer.onPositionChanged, this._drawers.changes).subscribe(() =>
-        first.call(this._ngZone.onMicrotaskEmpty).subscribe(() => this._validateDrawers()));
+    takeUntil.call(drawer.onPositionChanged, this._drawers.changes).subscribe(() => {
+      first.call(this._ngZone.onMicrotaskEmpty.asObservable()).subscribe(() => {
+        this._validateDrawers();
+      });
+    });
   }
 
   /** Toggles the 'mat-drawer-opened' class on the main 'md-drawer-container' element. */

src/lib/snack-bar/snack-bar-container.ts

@@ -165,11 +165,9 @@ export class MdSnackBarContainer extends BasePortalHost implements OnDestroy {
   private _completeExit() {
     // Note: we shouldn't use `this` inside the zone callback,
     // because it can cause a memory leak.
-    const onExit = this._onExit;
-
-    first.call(this._ngZone.onMicrotaskEmpty).subscribe(() => {
-      onExit.next();
-      onExit.complete();
+    first.call(this._ngZone.onMicrotaskEmpty.asObservable()).subscribe(() => {
+      this._onExit.next();
+      this._onExit.complete();
     });
   }
 }

src/lib/tooltip/tooltip.ts

@@ -372,7 +372,7 @@ export class MdTooltip implements OnDestroy {
       this._tooltipInstance.message = message;
       this._tooltipInstance._markForCheck();
 
-      first.call(this._ngZone.onMicrotaskEmpty).subscribe(() => {
+      first.call(this._ngZone.onMicrotaskEmpty.asObservable()).subscribe(() => {
         if (this._tooltipInstance) {
           this._overlayRef!.updatePosition();
         }