fix(cloud_firestore): fix returning of transaction result

packages/cloud_firestore/cloud_firestore/lib/src/firestore.dart

@@ -168,11 +168,15 @@ class FirebaseFirestore extends FirebasePluginPlatform {
   /// By default transactions are limited to 5 seconds of execution time. This
   /// timeout can be adjusted by setting the timeout parameter.
   Future<T> runTransaction<T>(TransactionHandler<T> transactionHandler,
-      {Duration timeout = const Duration(seconds: 30)}) {
+      {Duration timeout = const Duration(seconds: 30)}) async {
     assert(transactionHandler != null, "transactionHandler cannot be null");
-    return _delegate.runTransaction<T>((transaction) {
-      return transactionHandler(Transaction._(this, transaction));
+
+    T output;
+    await _delegate.runTransaction((transaction) async {
+      output = await transactionHandler(Transaction._(this, transaction));
     }, timeout: timeout);
+
+    return output;
   }
 
   /// Specifies custom settings to be used to configure this [FirebaseFirestore] instance.

packages/cloud_firestore/cloud_firestore_platform_interface/lib/src/method_channel/method_channel_firestore.dart

@@ -341,6 +341,8 @@ class MethodChannelFirebaseFirestore extends FirebaseFirestorePlatform {
       exception = e;
     });
 
+    // The #create call only resolves once all transaction attempts have succeeded
+    // or something failed.
     await channel.invokeMethod<T>('Transaction#create', <String, dynamic>{
       'firestore': this,
       'transactionId': transactionId,
@@ -349,7 +351,7 @@ class MethodChannelFirebaseFirestore extends FirebaseFirestorePlatform {
       exception = e;
     });
 
-    // The transaction is successful, cleanup the stream
+    // The transaction has completed (may have errored), cleanup the stream
     await subscription.cancel();
     _transactionStreamControllerHandlers.remove(transactionId);
 

packages/cloud_firestore/cloud_firestore_web/lib/cloud_firestore_web.dart

@@ -86,16 +86,15 @@ class FirebaseFirestoreWeb extends FirebaseFirestorePlatform {
   // }
 
   @override
-  Future<T> runTransaction<T>(TransactionHandler transactionHandler,
+  Future<T> runTransaction<T>(TransactionHandler<T> transactionHandler,
       {Duration timeout = const Duration(seconds: 30)}) async {
     try {
-      dynamic result = await _webFirestore.runTransaction((transaction) async {
+      await _webFirestore.runTransaction((transaction) async {
         return transactionHandler(
             TransactionWeb(this, _webFirestore, transaction));
       }).timeout(timeout);
-      // Workaround for 'Runtime type information not available for type_variable_local'
-      // See: https://github.com/dart-lang/sdk/issues/29722
-      return result as T;
+
+      return null;
     } catch (e) {
       throw getFirebaseException(e);
     }