Integer operations might not succeed (division by zero).

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 99 matching lines @@
   Constant fold(Constant left, Constant right) {
     if (left.isNum() && right.isNum()) {
       NumConstant leftNum = left;
       NumConstant rightNum = right;
       num foldedValue;
       if (left.isInt() && right.isInt()) {
         foldedValue = foldInts(leftNum.value, rightNum.value);
       } else {
         foldedValue = foldNums(leftNum.value, rightNum.value);
       }
-      assert(foldedValue != null);
+      // A division by 0 means that we might not have a folded value.
+      if (foldedValue === null) return null;
       if (left.isInt() && right.isInt() && !isDivide()) {
         assert(foldedValue is int);
         return new IntConstant(foldedValue);
       } else {
         return new DoubleConstant(foldedValue);
       }
     }
   }
 
   bool isDivide() => false;
@@ skipping 113 matching lines @@
     return new BoolConstant(left == right);
   }
 }
 
 class IdentityOperation implements BinaryOperation {
   const IdentityOperation();
   Constant fold(Constant left, Constant right) {
     return new BoolConstant(left == right);
   }
 }