Exceptions in transform block of Future doesn't appear

[DartBot]

This issue was originally filed by VadimTs...@gmail.com

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What steps will reproduce the problem?

1. Load and run https://gist.github.com/2905962

What is the expected output? What do you see instead?
Expected behaviour:
Consose output 'Before exception in transform block'
Stacktrace on error 'Error in transform block'
Instead I see only consose output 'Before exception in transform block'

What version of the product are you using? On what operating system?
dart-sdk revisions 8370 and 8478. On Windows 32bit.

[DartBot]

This comment was originally written by sammcca...@google.com

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The issue here is the strange behaviour of Future when a .then() is used without .handleException().

This causes Completer.completeException() to synchronously throws its argument.
In this case, the caller of completeException (on the transformed future completer) is an onComplete() handler of the original future. It can't do anything useful with the exception (in particular, it can't propagate it).

The best fix IMO is to make completion always asynchronous (this is a more general version of issue #3356).
That is, instead of throwing the exception, completeException() should do some portable equivalent of new Timer(0, (t) => throw exception);

A workaround for now is to handleException along with then(), or use onComplete.

[DartBot]

This comment was originally written by sammcca...@google.com

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I can write a patch reverting the changes to transform() and chain() from 2906010 until completions can be asynchronous.

[dgrove]

cc @sigmundch.
Added Area-Library, Triaged labels.

[sigmundch]

fixed in r8501

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Added Fixed label.