[firestore] Add FieldPath support to DocumentSnapshot and Query

lib/modules/firestore/utils/serialize.js

@@ -3,6 +3,7 @@
  */
 
 import DocumentReference from '../DocumentReference';
+import { DOCUMENT_ID } from '../FieldPath';
 import { DELETE_FIELD_VALUE, SERVER_TIMESTAMP_FIELD_VALUE } from '../FieldValue';
 import GeoPoint from '../GeoPoint';
 import Path from '../Path';
@@ -60,6 +61,10 @@ export const buildTypeMap = (value: any): FirestoreTypeMap | null => {
       type: 'fieldvalue',
       value: 'timestamp',
     };
+  } else if (value === DOCUMENT_ID) {
+    return {
+      type: 'documentid',
+    };
   } else if (type === 'boolean' || type === 'number' || type === 'string') {
     return {
       type,