TBR: Change BaseMatcher to Matcher in pub test file.

sdk/lib/_internal/pub/test/test_pub.dart

 // Copyright (c) 2013, 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.
 
 /// Test infrastructure for testing pub. Unlike typical unit tests, most pub
 /// tests are integration tests that stage some stuff on the file system, run
 /// pub, and then validate the results. This library provides an API to build
 /// tests like that.
 library test_pub;
 
@@ skipping 731 matching lines @@
         return new Pair(validator.errors, validator.warnings);
       });
     });
   }, "validating package");
 }
 
 /// A matcher that matches a Pair.
 Matcher pairOf(Matcher firstMatcher, Matcher lastMatcher) =>
    new _PairMatcher(firstMatcher, lastMatcher);
 
-class _PairMatcher extends BaseMatcher {
+class _PairMatcher extends Matcher {
   final Matcher _firstMatcher;
   final Matcher _lastMatcher;
 
   _PairMatcher(this._firstMatcher, this._lastMatcher);
 
   bool matches(item, Map matchState) {
     if (item is! Pair) return false;
     return _firstMatcher.matches(item.first, matchState) &&
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }