Matcher should always call describeMismatch of a nested macther

_Originally opened as dart-lang/sdk#14068_

_This issue was originally filed by wolf....@gmail.com_

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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.

Start by reproducing the failing unittest with a custom matcher nested inside isNot, then trace how the nested matcher’s describeMismatch result is handled. Done means the failure output includes a Which line with the nested mismatch description, matching the expected output in the issue.

Written by the indexing model from the issue text.