dart:io | Add Link.targetSync on all platforms.

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 9d324579344f92817c82285914032a9fd362986e..67869e3124591f27a0d97f9bbd5dbf7b9177552d 100644
--- a/runtime/bin/file_macos.cc
+++ b/runtime/bin/file_macos.cc
@@ -195,6 +195,25 @@ time_t File::LastModified(const char* name) {
 }
 
 
+char* File::LinkTarget(const char* pathname) {
+  struct stat link_stats;
+  if (lstat(pathname, &link_stats) != 0) return NULL;
+  if (!S_ISLNK(link_stats.st_mode)) {
+    errno = ENOENT;
+    return NULL;
+  }
+  size_t target_size = link_stats.st_size;
+  char* target_name = reinterpret_cast<char*>(malloc(target_size + 1));
+  size_t read_size = readlink(pathname, target_name, target_size + 1);
+  if (read_size != target_size) {
+    free(target_name);
+    return NULL;
+  }
+  target_name[target_size] = '\0';
+  return target_name;
+}
+
+
 bool File::IsAbsolutePath(const char* pathname) {
   return (pathname != NULL && pathname[0] == '/');
 }