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

runtime/bin/file.cc

Index: runtime/bin/file.cc
diff --git a/runtime/bin/file.cc b/runtime/bin/file.cc
index f457dd97f44c6f0bc950fdb0f4017294f20177a3..ae417e74891f7fb6a6b94d444cc59a121cebe742 100644
--- a/runtime/bin/file.cc
+++ b/runtime/bin/file.cc
@@ -439,8 +439,32 @@ void FUNCTION_NAME(File_CreateLink)(Dart_NativeArguments args) {
       Dart_SetReturnValue(args, err);
     }
   } else  {
-    OSError os_error(-1, "Invalid argument", OSError::kUnknown);
-    Dart_Handle err = DartUtils::NewDartOSError(&os_error);
+    Dart_Handle err = DartUtils::NewDartArgumentError(
+        "Non-string argument to Link.create");
+    if (Dart_IsError(err)) Dart_PropagateError(err);
+    Dart_SetReturnValue(args, err);
+  }
+  Dart_ExitScope();
+}
+
+
+void FUNCTION_NAME(File_LinkTarget)(Dart_NativeArguments args) {
+  Dart_EnterScope();
+  if (Dart_IsString(Dart_GetNativeArgument(args, 0))) {
+    const char* name =
+        DartUtils::GetStringValue(Dart_GetNativeArgument(args, 0));
+    char* target = File::LinkTarget(name);
+    if (target == NULL) {
+      Dart_Handle err = DartUtils::NewDartOSError();
+      if (Dart_IsError(err)) Dart_PropagateError(err);
+      Dart_SetReturnValue(args, err);
+    } else {
+      Dart_SetReturnValue(args, DartUtils::NewString(target));
+      free(target);
+    }
+  } else {
+    Dart_Handle err = DartUtils::NewDartArgumentError(
+        "Non-string argument to Link.target");
     if (Dart_IsError(err)) Dart_PropagateError(err);
     Dart_SetReturnValue(args, err);
   }