Fix failing unit tests.

Author: crisbeto

src/lib/core/overlay/overlay-ref.ts

@@ -38,7 +38,10 @@ export class OverlayRef implements PortalHost {
   }
 
   dispose(): void {
-    this._state.positionStrategy.dispose();
+    if (this._state.positionStrategy) {
+      this._state.positionStrategy.dispose();
+    }
+
     this._detachBackdrop();
     this._portalHost.dispose();
   }

src/lib/core/overlay/overlay.spec.ts

@@ -251,5 +251,6 @@ class FakePositionStrategy implements PositionStrategy {
     return Promise.resolve();
   }
 
+  dispose() {}
 }