Attempt at fixing https://github.com/dart-lang/tools/issues/2202.

[d-markey]

Hello,

I experienced an issue when using a MCP server (using dart_mcp which uses this package). Worked well on VM, but when I switched to Web, MCP initialization never returned. As it turns out, the target MCP server does not support CORS (eg. https://api.githubcopilot.com/mcp/) and it seems the error never flows back to the code that initializes the MCP server, so the corresponding future never completes. This is tracked in issue #2202.

I've added a test "can send a message and receive an error" that mimics the behavior I observed: the test times out without this fix. The fix consists in forwarding the error to both the client and the server. As it turns out, the Peer never called _clientIncomingForwarder.addError() which seems odd?

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

[gemini-code-assist]

Code Review

This pull request effectively addresses an issue where pending requests on a Peer would hang indefinitely if the underlying communication channel produced an error. The fix correctly propagates the channel error to both the client and server components of the Peer, ensuring that pending client requests are properly terminated. The introduction of a new test case successfully validates this fix by simulating a channel error and asserting the expected failure of a pending request. Additionally, enabling web platform testing is a valuable improvement. The code changes, including some nice formatting improvements, are clean, well-tested, and follow Dart best practices.