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

runtime/bin/file_linux.cc

Index: runtime/bin/file_linux.cc
diff --git a/runtime/bin/file_linux.cc b/runtime/bin/file_linux.cc
index df43d2d86c9788693a7b9b563123a1160b2963e3..abbe9cce2ac942ffd242da705d6251059738b0f5 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -160,6 +160,12 @@ 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) {