Fix JS deallocation on load errors

Summary:Fixes facebook#6281

The JS executor is invalidated from `[RCTBatchedBridger stopLoadingWithError:]`
which prevented another blocks that depended on the executor to run in the bridge
invalidation.

Reviewed By: javache

Differential Revision: D3018299

fb-gh-sync-id: 4f482b9b697bfabd24b405398d25c72b9e1a3c84
shipit-source-id: 4f482b9b697bfabd24b405398d25c72b9e1a3c84

React/Executors/RCTJSCExecutor.m

@@ -388,6 +388,11 @@ - (void)invalidate
 #if RCT_DEV
   [[NSNotificationCenter defaultCenter] removeObserver:self];
 #endif
+}
+
+- (void)dealloc
+{
+  [self invalidate];
 
   [_context performSelector:@selector(invalidate)
                    onThread:_javaScriptThread
@@ -396,11 +401,6 @@ - (void)invalidate
   _context = nil;
 }
 
-- (void)dealloc
-{
-  [self invalidate];
-}
-
 - (void)flushedQueue:(RCTJavaScriptCallback)onComplete
 {
   // TODO: Make this function handle first class instead of dynamically dispatching it. #9317773