Fix crashes on debug windows

runtime/bin/file.h

Index: runtime/bin/file.h
diff --git a/runtime/bin/file.h b/runtime/bin/file.h
index c30c02d8bf2d68874601cc4d073d9f09644543e2..cef6f0a56594ce645dfadce15ba269ec8ada5b68 100644
--- a/runtime/bin/file.h
+++ b/runtime/bin/file.h
@@ -162,6 +162,10 @@ class File : public ReferenceCounted<File> {
   // Like ScopedOpen(), but no API scope is needed.
   static File* Open(const char* path, FileOpenMode mode);
 
+  // Caution! On Windows, the static functions below may call
+  // Dart_ScopeAllocate() to do string conversions! If you call these functions
+  // without a scope, they will fail on Windows!

[siva, 2016/10/18 17:32:20]

GetType is in this list but it does not call Dart_ScopeAllocate after this CL,
maybe it should be moved out of the list?

I suppose Dart_ScopeAllocate was being used in order to make the
deletion of the allocated wide char string easy. 
It is useful to be able to call these file utility functions without
having to set up a Dart Scope. Could we use the C++ destructor
mechanism to achieve the same?

Something like :
File::Type File::GetType(const char* pathname, bool follow_links) {
  MutiByteToWideChar convert(pathname);
  const wchar_t* name = convert.string();
  ...
  ...
  return result;
}

The destructor of MultiByteToWideChar takes care of deleting the allocated
string.

+
   // Create a file object for the specified stdio file descriptor
   // (stdin, stout or stderr).
   static File* OpenStdio(int fd);