fs, handle_wrap: use Loop(), not uv_default_loop()

Using the default loop will fail in unexpected and interesting ways when
isolates are used.

src/handle_wrap.cc

@@ -89,7 +89,7 @@ Handle<Value> HandleWrap::Ref(const Arguments& args) {
   }
 
   wrap->unref = false;
-  uv_ref(uv_default_loop());
+  uv_ref(Loop());
 
   return v8::Undefined();
 }

src/node_file.cc

@@ -234,7 +234,7 @@ struct fs_req_wrap {
     uv_fs_t* req = &req_wrap->req_;                               \
     req->result = r;                                              \
     req->path = NULL;                                             \
-    req->errorno = uv_last_error(uv_default_loop()).code;         \
+    req->errorno = uv_last_error(Loop()).code;                    \
     After(req);                                                   \
   }                                                               \
   return scope.Close(req_wrap->object_);