Better runtime type checks.

frog/lib/num.dart

 // Copyright (c) 2011, 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.
 
 // Dart core library.
 
 interface num extends Comparable, Hashable {
   // Arithmetic operations.
   num operator +(num other);
   num operator -(num other);
@@ skipping 26 matching lines @@
   num truncate();
 
   int toInt();
   double toDouble();
 
   String toStringAsFixed(int fractionDigits);
   String toStringAsExponential(int fractionDigits);
   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);
   int operator ^(int other);
   int operator ~();
   int operator <<(int shiftAmount);
   int operator >>(int shiftAmount);
 
   // bit operation missing in corelib?
   int operator >>>(int shiftAmount);
 }