Add File.copy(Sync) to dart:io.

runtime/bin/file_macos.cc

Index: runtime/bin/file_macos.cc
diff --git a/runtime/bin/file_macos.cc b/runtime/bin/file_macos.cc
index 46c77cb40c86cc8147d82c6b302eacf79f971a2c..371809743d035d872cfd2ebf484864eaeb98c0ad 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -9,6 +9,7 @@
 
 #include <errno.h>  // NOLINT
 #include <fcntl.h>  // NOLINT
+#include <copyfile.h>  // NOLINT
 #include <sys/stat.h>  // NOLINT
 #include <unistd.h>  // NOLINT
 #include <libgen.h>  // NOLINT
@@ -221,6 +222,19 @@ bool File::RenameLink(const char* old_path, const char* new_path) {
 }
 
 
+bool File::Copy(const char* old_path, const char* new_path) {
+  File::Type type = File::GetType(old_path, true);
+  if (type == kIsFile) {
+    return copyfile(old_path, new_path, NULL, COPYFILE_DATA) == 0;
+  } else if (type == kIsDirectory) {
+    errno = EISDIR;
+  } else {
+    errno = ENOENT;
+  }
+  return false;
+}
+
+
 off64_t File::LengthFromPath(const char* name) {
   struct stat st;
   if (TEMP_FAILURE_RETRY(stat(name, &st)) == 0) {