Better runtime type checks.

frog/lib/num.dart

Index: frog/lib/num.dart
diff --git a/frog/lib/num.dart b/frog/lib/num.dart
index 431b09bdfbe021b29b1e285d6a8658598ca1e84a..bf40a12ba9ae8ee21eabcdefab6024ff8039f022 100644
--- a/frog/lib/num.dart
+++ b/frog/lib/num.dart
@@ -44,6 +44,9 @@ interface num extends Comparable, Hashable {
   String toStringAsPrecision(int precision);
   String toRadixString(int radix);
 
+  // TODO(jmesserly): we need to do something in the Frog type system to know
+  // to know that most int operations are closed over integers.
+
   // TODO(jimhug): Bit-operations stolen from int
   int operator &(int other);
   int operator |(int other);