[windows] Make most file_win.cc functions use malloc for string conversions.

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index a3dcd7fcb7cb43f2be388129ebd456c25096c1d4..828db7684948e346e750c76ed8a4d0ac54f531c7 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -92,7 +92,7 @@ void FUNCTION_NAME(File_Open)(Dart_NativeArguments args) {
   // reading. This is to prevent the opening of directories as
   // files. Directories can be opened for reading using the posix
   // 'open' call.
-  File* file = File::ScopedOpen(filename, file_mode);
+  File* file = File::Open(filename, file_mode);
   if (file != NULL) {
     Dart_SetReturnValue(args,
                         Dart_NewInteger(reinterpret_cast<intptr_t>(file)));
@@ -677,7 +677,7 @@ CObject* File::OpenRequest(const CObjectArray& request) {
     File::DartFileOpenMode dart_file_mode =
         static_cast<File::DartFileOpenMode>(mode.Value());
     File::FileOpenMode file_mode = File::DartModeToFileMode(dart_file_mode);
-    file = File::ScopedOpen(filename.CString(), file_mode);
+    file = File::Open(filename.CString(), file_mode);
     if (file != NULL) {
       return new CObjectIntptr(
           CObject::NewIntptr(reinterpret_cast<intptr_t>(file)));