Change handling of dart-ext: URIs for native extensions.

runtime/bin/extensions.cc

Index: runtime/bin/extensions.cc
diff --git a/runtime/bin/extensions.cc b/runtime/bin/extensions.cc
index bdfd5878d0bed973f20bcfa36a9a6d00d44e1f69..7b7f33ba0ba8eb11abaf1e5e19e9be6958127093 100644
--- a/runtime/bin/extensions.cc
+++ b/runtime/bin/extensions.cc
@@ -18,6 +18,10 @@ namespace bin {
 
 Dart_Handle Extensions::LoadExtension(const char* extension_path,
                                       Dart_Handle parent_library) {
+  if (strncmp(extension_path, "http://", 7) == 0) {
+    return Dart_NewApiError("Cannot load native extensions over http:");
+  }
+
   char* library_path = strdup(extension_path);
 
   if (library_path == NULL) {