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 ce0ef27bb26c5ea84f21e13424b345098c4d2f4e..488bafb7a2eaf72cba0ca64c4c9080091a74021d 100644
--- a/runtime/bin/file_linux.cc
+++ b/runtime/bin/file_linux.cc
@@ -159,6 +159,12 @@ 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) {