Add support for Expect.equals in the Leg core library.

frog/tests/leg_only/src/LoopIfPhiTest.dart

 // Copyright (c) 2011, 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.
 
-void expectEquals(var expected, var actual) {
-  if (expected == actual) {
-  } else {
-    print("Actual does not match expected");
-    throw actual;
-  }
-}
-
 void main() {
   var prev = -1;
   for (int i = 0; i < 2; i++) {
     var x = i;
     if (prev != -1) {
-      expectEquals(0, prev);
+      Expect.equals(0, prev);
     }
     prev = x;
   }
 }