Exception in DDC customer formatter logic

[vsmenon]

If I enable customer formatters, stop at a breakpoint, and eval / drill down on an object, I get an exception:

debugger.dart:524 Caught exception NoSuchMethodError: tried to call a non-function, such as null: 'dart.getImplements(...)'
 trace:
dart-sdk/lib/_internal/js_dev_runtime/private/debugger.dart 910:45       preview
dart-sdk/lib/_internal/js_dev_runtime/private/debugger.dart 483:63       preview
dart-sdk/lib/_internal/js_dev_runtime/private/debugger.dart 372:30       header
%3Canonymous%3E 1:5                                                      <fn>
packages/g200/player.dart 66:5    

This is on:

Google Chrome	86.0.4240.111 (Official Build) (x86_64)
Revision	b8c36128a06ebad76af51591bfec980224db5522-refs/branch-heads/4240@{#1290}
OS	macOS Version 10.15.7 (Build 19H2)
JavaScript	V8 8.6.395.17

and

Flutter 1.24.0-4.0.pre.144 • channel master • https://github.com/flutter/flutter.git
Framework • revision 470346f775 (3 hours ago) • 2020-10-29 09:56:28 -0700
Engine • revision 073263e39d
Tools • Dart 2.11.0 (build 2.11.0-260.0.dev)

This may be related to internal bug: b/171406503

@sigmundch @nshahan

[nshahan]

Can you walk me through how to enable them? I'm not sure how to repro. Are you using any chrome extensions?

[vsmenon]

We used to recommend enabling custom formatters. See here for general instructions: https://docs.google.com/document/d/1FTascZXT9cxfetuPRT2eXPQKXui4nWFivUnS_335T3U/preview

The Chrome team would like to disable them, however: https://bugs.chromium.org/p/chromium/issues/detail?id=1016755

Dart DevTools (or any IDE using the Dart protocol) should support the same functionality more robustly now.

That said, I took a look at the exception and a few things are going wrong. CL coming.

[vsmenon]

https://dart-review.googlesource.com/c/sdk/+/170022

[nshahan]

@vsmenon I'm trying to understand more about the failures you helpfully fixed. Were you able to reproduce them outside of a flutter build? I've reverted your change locally and I can't figure out how to make the formatter crash.

[vsmenon]

Yeah, I think could replicate in ddb as well. Turn on custom formatter, stop at a breakpoint, eval and drill down on objects. I was seeing exceptions in the console that matched internal bugs. I didn't ever get devtools itself to crash though.