Integer operations might not succeed (division by zero).

frog/leg/operations.dart

Index: frog/leg/operations.dart
diff --git a/frog/leg/operations.dart b/frog/leg/operations.dart
index f241d2565a63924f9756dc6ce8f364f8f8ee7a1e..0f5ea51f6da5595c823b0b0c52191e0a2d913875 100644
--- a/frog/leg/operations.dart
+++ b/frog/leg/operations.dart
@@ -117,7 +117,8 @@ class ArithmeticNumOperation implements BinaryOperation {
       } 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);