Convert most interfaces in dart:core to abstract classes.

lib/core/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.
 
 /**
  * Expect is used for tests that do not want to make use of the
  * Dart unit test library - for example, the core language tests.
  * Third parties are discouraged from using this, and should use
  * the expect() function in the unit test library instead for
  * test assertions.
  */
 class Expect {
-
   /**
    * Checks whether the expected and actual values are equal (using `==`).
    */
   static void equals(var expected, var actual, [String reason = null]) {
     if (expected == actual) return;
     String msg = _getMessage(reason);
     _fail("Expect.equals(expected: <$expected>, actual: <$actual>$msg) fails.");
   }
 
   /**
@@ skipping 259 matching lines @@
   }
 }
 
 typedef bool _CheckExceptionFn(exception);
 
 class ExpectException implements Exception {
   ExpectException(this.message);
   String toString() => message;
   String message;
 }