Support some language construct to be able to access private things from unit tests

[DartBot]

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

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Currently we need to do some hacks to be able to see private things in libraries from unit tests. It's all discussed in this thread:

https://groups.google.com/a/dartlang.org/forum/?fromgroups=#!topic/misc/0A0YnNILsDU

We have some hacks to be able to do it, but it would be wise to add some feature to the language so that we can do it in a clean way.

I would suggest something like being able to add a fragment on top of an existing library (similar to OSGi fragments) or a keyword similar to C++'s "friend".

[kasperl]

Added Area-Language, Triaged labels.

[DartBot]

This comment was originally written by ross.dart...@gmail.com

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In .NET this is done w/ an annotation instead of a keyword:

http://msdn.microsoft.com/en-us/library/system.runtime.compilerservices.internalsvisibletoattribute.aspx

[gbracha]

Set owner to @gbracha.
Added Accepted label.

[gbracha]

Added Duplicate label.
Marked as being merged into #1882.