initial commit: basic flow errors

lib/flow.js

@@ -21,3 +21,13 @@ declare type GoogleApiAvailabilityType = {
   isUserResolvableError?: boolean,
   error?: string
 };
+
+declare class FirebaseError {
+  message: string,
+  name: string,
+  code: string,
+  stack: string,
+  path: string,
+  details: string,
+  modifiers: string
+};