pkg/matcher: refactor with unittest v0.12 work

CHANGELOG.md

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+## 0.12.0
+
+* Moved a number of members to the
+[`unittest`](https://pub.dartlang.org/packages/unittest) package.

[nweiz, 2015/02/17 23:17:42]

Nit: indent +2. Also below.

[kevmoo, 2015/02/18 00:33:40]

Done.

+  * `TestFailure`, `ErrorFormatter`, `expect`, `fail`, and 'wrapAsync'

[nweiz, 2015/02/17 23:17:42]

Nit: add a period.

[kevmoo, 2015/02/18 00:33:40]

Done.

+  * `completes`, `completion`, `throws`, and `throwsA` Matchers.
+  * The `Throws` class.
+  * All of the `throws...Error` Matchers.

[nweiz, 2015/02/17 23:17:42]

Have you checked to be sure this nested list renders right? Markdown can be
finnicky with lists and spacing.

[kevmoo, 2015/02/18 00:33:40]

Yup

+
+* Removed `FailureHandler`, `DefaultFailureHandler`,
+`configureExpectFailureHandler`, and `getOrCreateExpectFailureHandler`.
+Now that `expect` is in the `unittest` package, these are no longer needed.
+
 ## 0.11.4+1
 
 * Correctly match and print `String`s containing characters that must be