dart:io | Add Link class, as sibling to File and Directory.

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 7506cbfbb260ac066bf8121c51c533dbf853d369..9d324579344f92817c82285914032a9fd362986e 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -162,6 +162,12 @@ bool File::Create(const char* name) {
 }
 
 
+bool File::CreateLink(const char* name, const char* target) {
+  int status = TEMP_FAILURE_RETRY(symlink(target, name));
+  return (status == 0);
+}
+
+
 bool File::Delete(const char* name) {
   int status = TEMP_FAILURE_RETRY(remove(name));
   if (status == -1) {