Error out on compile-time constants if equality is tested on bad types.

frog/leg/operations.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.
 
 interface Operation {
   
 }
 
 interface UnaryOperation extends Operation {
   /** Returns [:null:] if it was unable to fold the operation. */
@@ skipping 249 matching lines @@
 class EqualsOperation implements BinaryOperation {
   const EqualsOperation();
   Constant fold(Constant left, Constant right) {
     if (left.isNum() && right.isNum()) {
       // Numbers need to be treated specially because: NaN != NaN, -0.0 == 0.0,
       // and 1 == 1.0.
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       return new BoolConstant(leftNum.value == rightNum.value);
     }
+    if (left.isConstructedObject()) {
+      // Unless we know that the user-defined object does not implement the
+      // equality operator we cannot fold here.
+      return null;
+    }
     return new BoolConstant(left == right);
   }
 }
 
 class IdentityOperation implements BinaryOperation {
   const IdentityOperation();
   Constant fold(Constant left, Constant right) {
     return new BoolConstant(left == right);
   }
 }