Matcher should always call describeMismatch of a nested macther

[DartBot]

Originally opened as dart-lang/sdk#14068

This issue was originally filed by [email protected]

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What steps will reproduce the problem?

1. Create a Matcher with has a describeMismatch method
2. Create a falling unittest with the Matcher in the isNot-Matcher: isNot(myMatcher)

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

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Expected: not <my description>   Actual: <value>    Which: <my mismatch description>

Actual:

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Expected: not <my description>   Actual: <value>

What version of the product are you using? On what operating system? Dart VM version: 0.8.1.2_r28355 (Tue Oct 08 06:29:27 2013) on "windows_ia32"

Please provide any additional information below. All matcher that contain other matcher should add the nested description. At least isNot is missing that.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/2156198?v=3" align="left" width="48" height="48"hspace="10"> Comment by kasperl

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Added Area-UnitTest, Triaged labels.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Added Pkg-Unittest label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Removed Area-UnitTest label. Added Area-Pkg label.

[DartBot]

<img src="https://avatars.githubusercontent.com/u/17034?v=3" align="left" width="48" height="48"hspace="10"> Comment by kevmoo

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Removed Pkg-Unittest label. Added Pkg-matcher label.

[SandPod]

I am writing my own matcher currently, and I just realized that my neat formatting is thrown out the window as soon as I combine my matcher with an isNot(...) matcher. It might not be trivial to solve but it would be nice to be able to provide some message here.