feat: add preliminary web support

[Frank3K]

Goal

This pull request refactors the platform-specific implementation of the bugsnag_flutter client to use a more modular and extensible approach, enabling preliminary support for web.

Note that our goal is to add MVP support for Flutter on web. Only the methods and properties that are actually used in our applications are supported; other methods throw UnimplementedErrors.

Installation

In order to add support for bugsnag on Flutter web, the following script needs to be added to the index.html of the application (taken from the docs).

<script src="//d2wy8f7a9ursnm.cloudfront.net/v8/bugsnag.min.js"></script>

Design

The core change splits the monolithic client.dart into a platform-conditional architecture using Dart's conditional
imports:

// client.dart
import 'client_stub.dart'
    if (dart.library.io) 'client_io.dart'
    if (dart.library.js_interop) 'client_web.dart';

At compile time, Dart selects the correct implementation file based on the target platform's available libraries. Each file exports the same two top-level factory functions — platformStart() and platformAttach() — which the Bugsnag facade calls to obtain a platform-specific BugsnagClient.

  graph TD
      subgraph "Public API"
          C["BugsnagClient<br/><i>abstract class</i>"]
      end

      subgraph "Platform resolution"
          C -->|"dart.library.io"| D["client_io.dart<br/><b>ChannelClient</b>"]
          C -->|"dart.library.js_interop"| E["client_web.dart<br/><b>WebClient</b>"]
          C -->|"fallback"| F["client_stub.dart<br/><i>throws UnsupportedError</i>"]
      end

      subgraph "Native layer"
          D -->|"MethodChannel"| G["Android/iOS<br/>Bugsnag SDK"]
          E -->|"dart:js_interop"| H["@bugsnag/browser<br/>JS library"]
      end

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File responsibilities

• client.dart — Abstract BugsnagClient interface, DelegateClient mixin, Bugsnag facade. Fully platform-agnostic — no dart:io or dart:js_interop imports.
• client_io.dart — ChannelClient — existing Android/iOS implementation. Communicates with native SDKs via Flutter MethodChannel. Selected when dart.library.io is available.
• client_web.dart — WebClient — new web implementation. Calls @bugsnag/browser via dart:js_interop bindings (@JS() annotations). Selected when dart.library.js_interop is available.
• client_stub.dart — Default fallback. Both platformStart() and platformAttach() throw UnsupportedError.

How delegation works

The Bugsnag singleton uses the DelegateClient mixin, which holds a nullable _client reference. When bugsnag.start() or bugsnag.attach() is called, it invokes the conditionally-imported platformStart()/platformAttach() function, which returns a ChannelClient (IO) or WebClient (web). All subsequent API calls on bugsnag are forwarded to this inner client.

Web implementation details

WebClient uses dart:js_interop to call the @bugsnag/browser JS library that must already be loaded in the host page. Currently implemented: start, notify, setUser, resumeSession, and automatic unhandled error capture (via FlutterError.onError and PlatformDispatcher.instance.onError). Remaining API methods throw UnimplementedError and can be added incrementally.

Changeset

• Min dart SDK has been raised from 3.0.0 to 3.3.0.

Testing