testing only. remove before making PR

Author: Feiyang1

packages/firestore/src/core/version.ts

@@ -15,7 +15,8 @@
  * limitations under the License.
  */
 
-import firebase from '@firebase/app';
+//import firebase from '@firebase/app';
 
 /** The semver (www.semver.org) version of the SDK. */
-export const SDK_VERSION = firebase.SDK_VERSION;
+// export const SDK_VERSION = firebase.SDK_VERSION;
+export const SDK_VERSION = 'firebase.SDK_VERSION';