Move minitest.dart into the expect package.

pkg/expect/lib/minitest.dart

 // Copyright (c) 2016, 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.
 
 /// A minimal, dependency-free emulation layer for a subset of the
 /// unittest/test API used by the language and core library (especially HTML)
 /// tests.
 ///
 /// A number of our language and core library tests were written against the
 /// unittest package, which is now deprecated in favor of the new test package.
@@ skipping 161 matching lines @@
 
 /// A wrapper around an expectation function.
 ///
 /// This function is passed the actual value and should throw an
 /// [ExpectException] if the value doesn't match the expectation.
 class _Expectation {
   final _ExpectationFunction function;
 
   _Expectation(this.function);
 }