Support connecting to VM service via MessagePort / postMessage

[jonasfj]

Support for providing VM service protocol using a MessagePort.

• ?uri=messageport:<targetOrigin>
• devtools will do window.parent.postMessage({action: 'connect', port})

Then parent window is responsible for proxying messages into the MessagePort.

Note: MessagePort doesn't actually carry a disconnect / closed event, so in an ideal world we might want to allow the other end to send null to indicate end-of-stream. But I'm not sure we need it.

For context I'm trying to add the VM Service Protocol bits required in package:dartpad here: sdk/+/554920

AI description

Adds ?uri=messageport:<targetOrigin>, so a page embedding DevTools can provide the VM service connection over a MessagePort instead of a WebSocket URI.

• Why: DartPad runs its VM service in a web worker, so there is no WebSocket for DevTools to connect to.
• Protocol: on every connect, including reconnects and reloads, DevTools creates a MessageChannel and posts one port to the embedding page:

  window.parent.postMessage({action: 'connect', port}, targetOrigin, [port]);

  The embedding page connects the port to a VM service. Messages are JSON-RPC strings, or binary frames as Uint8Array. Only the latest port is used.
• Design: a URI scheme rather than a new query parameter, so routing, reconnecting and reloading keep working unchanged. The transport lives in devtools_app rather than in devtools_shared's connect, because that is published API and must not depend on web-only libraries.
• Not supported yet: DevTools extensions receive the messageport: URI but can't connect with it.
• Tests: VM tests for URI handling and the non-web stub, plus a browser test that plays both the embedding page and a fake VM service. CI only runs devtools_app tests on the VM, so I ran the browser test locally with --platform chrome and --platform chrome --wasm.

[kenzieschmoll]

CC @bkonyi

[gemini-code-assist]

Code Review

This pull request introduces support for proxying the VM service connection over a MessagePort, allowing DevTools to connect when embedded in an iframe. The implementation includes a web-specific connection handler, a stub for other platforms, and corresponding tests. The review feedback identifies several areas for improvement: adding a check for window.parent to prevent hanging when not embedded, implementing a graceful disconnection mechanism via null signals, validating the URI path to avoid runtime exceptions, and adding a timeout to the connection verification process to ensure robustness.