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

frog/tests/leg_only/src/LoopIfPhiTest.dart

Index: frog/tests/leg_only/src/LoopIfPhiTest.dart
diff --git a/frog/tests/leg_only/src/LoopIfPhiTest.dart b/frog/tests/leg_only/src/LoopIfPhiTest.dart
index bb652b7baa683b84d1b9113634ad033e3dbd783f..7d32e119486594c08cf5c27765e6485fcf36c68d 100644
--- a/frog/tests/leg_only/src/LoopIfPhiTest.dart
+++ b/frog/tests/leg_only/src/LoopIfPhiTest.dart
@@ -2,20 +2,12 @@
 // 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;
   }