Add hot reload capabilities to DartPad UI

[biggs0125]

This change integrates DDC hot reload into DartPad allowing for a faster iteration cycle and stateful reload.

The new reload workflow is currently limited to the main branch and only for Flutter applications. As the beta and stable branches of the SDK gain support for hot reload, we can expand this.

How it works:
This PR adds 2 new API entrypoints that invoke DDC with flags that produce JS using DDC's new module system. This new module system is what enables hot reload. Upon receiving the new code and loading them into the page as local blobs, the hotReload callback is invoked in the DDC embedder API. This triggers DDC to fetch the new file and reload the specified library, in this case the synthetic library created with the user's code. Once the hot reload is done, we trigger a reload event in Flutter via the 'reassemble' developer extension.

Hot reload doesn't work for all changes, some changes require a full restart. DDC has the ability to diff the new code provided to it against the kernel of a previous compilation. If it detects an unsafe change it will error. In order to facilitate this diffing, DartPad needs to pass the dill file from the last generation of the code. I've extended the request/response model to include a base64 encoded string of the relevant dill file. The previous dill will get stored in memory on the client and attached to the next request. The dill file only includes the user code library (since that's the only thing that can change) and is therefore fairly small.

I've intentionally left the API for the "old" DDC invocation as-is so that the new version of the API is backwards compatible. This effectively adds 2 new endpoints which should be a safe operation.

In testing this I realized that the reload requests were fairly slow because a lot of libraries were getting included in each compilation. To fix this, I've updated the pre-compiled code (flutter_sdk.js/flutter_sdk.dill) to include all the libraries that are always needed. This cut compilation times from ~2000ms -> ~500ms.

See videos of full workflow with these changes:
Full DartPad Hot Reload demo.webm
DartPad Hot Reload Error.webm

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

Coverage ✔️

File	Coverage

This check for test coverage is informational (issues shown here will not fail the PR).

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/samples/lib/brick_breaker.dart
pkgs/samples/lib/fibonacci.dart
pkgs/samples/lib/google_ai.dart
pkgs/samples/lib/hello_world.dart
pkgs/samples/lib/main.dart
pkgs/samples/lib/sunflower.dart

[johnpryan]

The new reload workflow is currently limited to the main branch and only for Flutter applications.

Has the code landed in main yet? I'm not able to get this running on the Flutter SDK's main channel,

compileNewDDC
FormatException: Could not find an option named "--new-reload-delta-kernel".
The Dart Development Compiler compiles Dart sources into a JavaScript module.

Here's my SDK version:

flutter --version
Flutter 3.29.0-1.0.pre.148 • channel main • https://github.com/flutter/flutter.git
Framework • revision b007899d3a (6 hours ago) • 2025-01-29 14:40:21 +0200
Engine • revision b007899d3a
Tools • Dart 3.8.0 (build 3.8.0-24.0.dev) • DevTools 2.42.0

[biggs0125]

The prerequisite change landed in the Dart SDK yesterday but a Dart roll has not happened into the Flutter SDK yet. We're trying to push one through today. Once that happens it'll be available on the main Flutter channel.

[johnpryan]

LGTM - let's not land this yet until we're ready.