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

frog/leg/operations.dart

Index: frog/leg/operations.dart
diff --git a/frog/leg/operations.dart b/frog/leg/operations.dart
index 8ad5427d01f1e2d320ded4f8f4d14d8bcd74c2a2..ff66987d2415e711220c0456508ea2924c093e01 100644
--- a/frog/leg/operations.dart
+++ b/frog/leg/operations.dart
@@ -267,6 +267,11 @@ class EqualsOperation implements BinaryOperation {
       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);
   }
 }