[Fuchsia] Fixes and cleanup for launchpad_vmo_from_file() deprecation

runtime/bin/extensions_fuchsia.cc

Index: runtime/bin/extensions_fuchsia.cc
diff --git a/runtime/bin/extensions_fuchsia.cc b/runtime/bin/extensions_fuchsia.cc
index cc80a5804fdd4dcc0f9800a658e89714cc4322ba..aea8dc3660d3ec079101cfecf398a25357d472d7 100644
--- a/runtime/bin/extensions_fuchsia.cc
+++ b/runtime/bin/extensions_fuchsia.cc
@@ -8,8 +8,10 @@
 #include "bin/extensions.h"
 
 #include <dlfcn.h>
+#include <fcntl.h>
 #include <launchpad/vmo.h>
 #include <magenta/dlfcn.h>
+#include <mxio/io.h>
 
 #include "platform/assert.h"
 
@@ -23,8 +25,14 @@ const char* kIsolateSnapshotInstructionsSymbolName =
     "_kDartIsolateSnapshotInstructions";
 
 void* Extensions::LoadExtensionLibrary(const char* library_file) {
-  mx_handle_t vmo = launchpad_vmo_from_file(library_file);
-  if (vmo <= 0) {
+  int fd = open(library_file, O_RDONLY);
+  if (fd < 0) {
+    return NULL;
+  }
+  mx_handle_t vmo;
+  mx_status_t status = mxio_get_vmo(fd, &vmo);
+  close(fd);
+  if (status != MX_OK) {
     return NULL;
   }
   return dlopen_vmo(vmo, RTLD_LAZY);