The error for using dart:io is unhelpful

[devoncarew]

From @lukechurch on January 5, 2015 12:30

"Error compiling:
[500 Internal Server Error] [error, line 1] Library not found 'dart:io'."

Should be something along the lines of

"The dart:io library cannot be used in a browser context. Please see support page for common work arounds."

Lets chat about where we put in the fix.

Copied from original issue: devoncarew/dartpad_ui#14

[devoncarew]

It now reads:

There was an issue when compiling to JavaScript:
[error, line 1] Library not found 'dart:io'.

Which is an intermediate step towards the above solution.

[sethladd]

We should prevent the Run button from being clickable, if the code somehow imports dart:io.

[devoncarew]

The error has been cleaned up quite a bit. We can get some more info from the backend when analyzing, to be able to recognize a sample that directly or transitively imports dart:io.

[Sfshaza]

I am confused.

I am able to read a file in the darrrrt codelab, which depends on dart:html. So I expected that creating a simple example that relies on dart:io would also be able to read a file.

I tried it and got a confusing error:

CONSOLE
Error compiling to JavaScript:
[error on line 1] Library not found 'dart:io'.
[warning on line 7] Cannot resolve 'File'.

If dart:io is not supported, I would expect a clearer message saying that dart:io is not supported.

And why can I read a file in the Pirate codelab (dart:html) but not dart:io?

You can see my example here:
https://dartpad.dartlang.org/75e9c57f714e8a51ef21

Here is the Pirate codelab, which reads a JSON file:
https://dartpad.dartlang.org/9ccf6694c91268e78e79

[devoncarew]

Yeah, the net-net is that dart:io is not supported, and there's no real path to supporting it in dartpad.

We can do something like scrape the error message that comes back from dart2js in order to display a better error message.

You can use the dart:html apis to read a file because we compile your app to javascript and execute it in the browser (in an embedded iframe). We can't run a dart:io app in the browser, and we don't have a sandboxed environment to be able to execute it on a server :(

[devoncarew]

The action item here is to improve the error message in order to make it clear that the reason the app failed to run is because the dart:io library is not supported in dartpad.

[Sfshaza]

Yes, thanks.

[kwalrath]

Still an issue. You can even import 'dart:io' and get reasonable looking docs, which makes you think it'll be OK to use...

[matanlurey]

Another user ran into this.

[RedBrogdon]

Just opened a related but broader issue: #1000. I'm going to close this one as a duplicate of the new one.