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

runtime/bin/file_android.cc

Index: runtime/bin/file_android.cc
diff --git a/runtime/bin/file_android.cc b/runtime/bin/file_android.cc
index 1ff855ada4a46c2dc0eee0011aee6c885d21aaea..bdb8ac1a0a3f85cd97c5b9be18d2cb5d41a6bc72 100644
--- a/runtime/bin/file_android.cc
+++ b/runtime/bin/file_android.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) {