dart2js: try-catch block fails in IE9 and Firefox

[DartBot]

This issue was originally filed by @bp74

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What steps will reproduce the problem?

import('dart:html');

void main() {
  AudioContext audioContext;
  
  try {
    audioContext = new AudioContext();
  }
  catch(final error) {
    print(error);
  }
}

What is the expected output? What do you see instead?

The code above should throw an exception in IE9 and Firefox, because "AudioContext" is not available in this browsers. Unfortunately the JavaScript generated by dart2js fails with an error. The same code compiled with "frog" works fine. This is the error you get in dart2js:

SCRIPT5007: Unable to get value of the property 'operator$index$1': object is null or undefined

Attached to this issue is a picture of the JavaScript debugger, showing the code where the error happens.

What version of the product are you using? On what operating system?

Dart Editor 7916 32 bit, Windows 7 64 bit

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Attachment:
debug.png (56.08 KB)

[DartBot]

This comment was originally written by @bp74

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added another picuture showing the call stack.

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Attachment:
debug2.png (53.96 KB)

[DartBot]

This comment was originally written by @bp74

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addedd another picture showing the line of code causing the problem.

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Attachment:
debug3.png (60.17 KB)

[kasperl]

Added Area-Dart2JS, Triaged labels.

[kasperl]

This is an interesting case because we do have an AudioContext class in dart:html. Do you have any insights on this, Stephen? Is the fact that this "works" in frog by design and is it something we've missed in dart2js?

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cc @rakudrama.

[rakudrama]

The code equivalent to unwrapException in the frog runtime is written in JavaScript.
The dart2js version is written in Dart using a few 'JS' forms for low level access.
The difference in behaviour is due to the JavaScript version using [] in JavaScript, where the Dart version uses the Dart indexing operator: jsArguments[0].

Change this to JS('Object', '#[0]', jsArguments) or something similar.

Why does this matter?
I have not verified, but I believe IE is using either a plain object or a special object to hold the arguments.
It is the difference between (in JavaScript)
   ['Foo', 123]
and
   {'length':2, '0':'Foo', '1':123}
The second is not an Array, but it is 'Array-like' - it works most of the time.

I expect we will see more cases like this, e.g. the value of the JavaScript 'arguments' special variable is usually not an instance of Array, so it is copied to an Array with Array.prototype.slice.call(arguments)

[peter-ahe-google]

Thank you, Stephen. As always, very enlightening.

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Set owner to @peter-ahe-google.
Added Accepted label.

[kasperl]

CL for review: https://chromiumcodereview.appspot.com/10445032/.

[kasperl]

Fixed in r7991. Bernhard, if it's not too much to ask, it would be great if you could verify that this solves your issue.

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Set owner to @kasperl.
Added Fixed label.

[DartBot]

This comment was originally written by @bp74

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Of course i'm happy to test it! I have good and bad news for you :)

The bug in this issue is fixed, now the "catch" block is called - which is good!

The bad thing is that a new issue came up:
SCRIPT5007: Unable to get value of the property 'charCodeAt$1': object is null or undefined

But i think we should open a new issue for this. The problem is that the "error" variable in the example above cannot be converted to a String. I have attached three new pictures with the call stack.

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Attachments:
shot1.png (54.43 KB)
shot2.png (55.90 KB)
shot3.png (52.78 KB)

[DartBot]

This comment was originally written by @bp74

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Sorry, forgot to say i have tested with r7992.

[kasperl]

Thanks, Bernhard. Feel free to open a new issue. Looking forward to getting to the bottom of this!