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 6ffac3d5ea55b837e12333c3401939e980e4acde..f85e1c42a0ce489365c723543b2d6112f5a4e848 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<int64_t>& handles = isolate_data->unclosed_handles;
+  for (auto it = handles.begin(); it != handles.end(); ++it) {
+    MojoClose(static_cast<MojoHandle>(*it));
+  }
+  handles.clear();
 }