IsolateSpawnException should expose more metadata

[nex3]

Currently, IsolateSpawnException only exposes a [toString] method (which is poorly formatted -- see issue #12689). This makes it very difficult to report isolate errors, such as those from "pub serve", in a user-friendly manner.

For an error caused by a library failing to load, it would be useful to have, in order of descending usefulness:

* The URI of the library that failed to load.

• The URIs of libraries that transitively imported that library.
• The line and position of each of those import statements.
• Information about the HTTP or filesystem error that caused the library load to fail.

[lrhn]

Added Area-Library label.

[nex3]

Marked this as blocking #17200.

[kevmoo]

Er, blocking dart-lang/pub#896

[lrhn]

This is not information that we can generally have. If dart2js fails to spawn an isolate, it's either because the JS files didn't exist or it had an error.

I think each platform should have a best-effort approach to reporting the errors, but it's not something we can generally add as fields of a the error.

[nex3]

I'm fine if there are fields that are null in dart2js. But the VM at least has access to a bunch of information that we need, and the only way we're currently able to get it is by parsing strings, which means we get broken every so often when IsolateSpawnException.toString() changes.

[floitschG]

Reassigning to VM.

This kind of information would need to come from the VM/embedder.

[matanlurey]

dart-lang/pub#896 is closed, so I assume we can close this.