Dart: Better handle leak checks. close() is async.

mojo/dart/embedder/dart_controller.cc

Index: mojo/dart/embedder/dart_controller.cc
diff --git a/mojo/dart/embedder/dart_controller.cc b/mojo/dart/embedder/dart_controller.cc
index 03bd10f67271fa7c95e1bd4761507eb38e776d9a..91775a0c6ea63a05e429e289cbaa514103ca0ec6 100644
--- a/mojo/dart/embedder/dart_controller.cc
+++ b/mojo/dart/embedder/dart_controller.cc
@@ -13,6 +13,7 @@
 #include "mojo/dart/embedder/builtin.h"
 #include "mojo/dart/embedder/dart_controller.h"
 #include "mojo/dart/embedder/isolate_data.h"
+#include "mojo/public/c/system/core.h"
 
 namespace mojo {
 namespace dart {
@@ -429,6 +430,13 @@ static void UnhandledExceptionCallback(Dart_Handle error) {
     // for clients if we pass any error string here instead.
     isolate_data->callbacks.exception.Run(error);
   }
+
+  // Close handles generated by the isolate.
+  std::set<MojoHandle>& handles = isolate_data->unclosed_handles;
+  for (auto it = handles.begin(); it != handles.end(); ++it) {
+    MojoClose((*it));
+  }
+  handles.clear();
 }