Expose Timestamp in SDK

Author: op

src/sdk.js

@@ -4,6 +4,7 @@ var MockFirestore = require('./firestore');
 var MockFieldValue = require('./firestore-field-value');
 var MockMessaging = require('./messaging');
 var MockStorage = require('./storage');
+var MockTimestamp = require('./timestamp');
 
 var EmailAuthProvider = function() {
   this.providerId = EmailAuthProvider.PROVIDER_ID;
@@ -77,6 +78,7 @@ function MockFirebaseSdk(createDatabase, createAuth, createFirestore, createStor
     return createFirestore ? createFirestore() : new MockFirestore();
   }
   MockFirebaseFirestore.FieldValue = MockFieldValue;
+  MockFirebaseFirestore.Timestamp = MockTimestamp;
 
   function MockFirebaseStorage() {
     return createStorage ? createStorage() : new MockStorage();