Update unittest to new package layout.

pkg/unittest/lib/src/expect.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * This is the main assertion function. It asserts that [actual]
  * matches the [matcher]. [matcher] is optional and defaults to isTrue,
  * so expect can be used with a single predicate argument. [reason]
  * is optional and is typically not supplied if a reasonable matcher is
  * explicitly provided, as a reason can be generated from the matcher.
@@ skipping 126 matching lines @@
  * formatter is returned; this allows custom expect handlers to easily
  * get a reference to the default formatter.
  */
 ErrorFormatter configureExpectFormatter([ErrorFormatter formatter = null]) {
   if (formatter == null) {
     formatter = _defaultErrorFormatter;
   }
   return _assertErrorFormatter = formatter;
 }