Add native extensions for the Dart shell to mac and windows

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 d81e670eef10218fe7bc68228faa4d0d88a9083c..0ede8297c5dec3cbfb78f055be80cdcafd86172d 100644
--- a/runtime/bin/extensions_win.cc
+++ b/runtime/bin/extensions_win.cc
@@ -4,11 +4,14 @@
 
 #include "bin/extensions.h"
 
-#include "include/dart_api.h"
-#include "platform/assert.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);
+  return lib_handle;
+}
 
-Dart_Handle Extensions::LoadExtension(const char* extension_url,
-                                      Dart_Handle parent_library) {
-  UNIMPLEMENTED();
-  return NULL;
+void* Extensions::ResolveSymbol(void* lib_handle, const char* symbol) {
+  return GetProcAddress(reinterpret_cast<HMODULE>(lib_handle), symbol);
 }