Provide better tools to detect Errors at runtime

[vsmenon]

In general, Error objects aren't treated any differently from Exception objects - developers typically write code that can catch either:

try {
  String s = null;
  s.length;
} catch(e) {
  print(e);
}

Given that our compilers can (under certain flags) assume that Error would not be thrown (see #34044), we should provide better support in our to detect when they occur at runtime during development and testing.

E.g., DDC might provide (or default to) a mode where Error objects cannot be caught (accommodating for frameworks). DDT might provide a 'Pause on Error' option.

@jmesserly @alan-knight @sigmundch

[vsmenon]

This actually lines up well with our existing documentation on Error:

https://api.dartlang.org/dev/2.0.0-dev.69.5/dart-core/Error-class.html

Specifically:

These are not errors that a caller should expect or catch - if they occur, the program is erroneous, and terminating the program may be the safest response.

[alan-knight]

For DDT we already pause on Errors, e.g. RangeError. Are we talking about a change to errors where they're not throwing? It would be pretty easy to have DDC compile errors to include a "debugger" statement, possibly conditional.

[vsmenon]

I meant differentiating between pausing on a Dart Error vs pausing on anything else (e.g., Exception).

[leafpetersen]

I wonder if having a debug mode flag to implicitly treat } catch(e) { as } on Exception catch(e) { might be useful? That would mean that unless you are explicitly opting in to catching Error in some way, you won't accidentally swallow Error exceptions.

[vsmenon]

Yes, I think we want something like that - although perhaps catching everything except errors (e.g., plain Objects):

catch(e) {
  if (e is Error) rethrow;
  ...
}