Change lookup path for Dart native extensions shared libraries.

runtime/bin/extensions_win.cc

Index: runtime/bin/extensions_win.cc
diff --git a/runtime/bin/extensions_win.cc b/runtime/bin/extensions_win.cc
index 0ede8297c5dec3cbfb78f055be80cdcafd86172d..239c0bf2c711a992310ad046b6b6646cb55fb796 100644
--- a/runtime/bin/extensions_win.cc
+++ b/runtime/bin/extensions_win.cc
@@ -4,11 +4,12 @@
 
 #include "bin/extensions.h"
 
-void* Extensions::LoadExtensionLibrary(const char* library_name) {
-  const char* strings[3] = { library_name, ".dll", NULL };
-  char* library_path = Concatenate(strings);
-  void* lib_handle = LoadLibrary(library_path);
-  free(library_path);
+void* Extensions::LoadExtensionLibrary(const char* library_path,
+                                       const char* extension_name) {
+  const char* strings[5] = { library_path, "\\", extension_name, ".dll", NULL };
+  char* library_file = Concatenate(strings);
+  void* lib_handle = LoadLibrary(library_file);
+  free(library_file);
   return lib_handle;
 }