Fix issue 100:

lib/double.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.
 
 class Double implements double {
   factory Double.fromInteger(int value)
       native "Double_doubleFromInteger";
   int hashCode() {
     try {
       return toInt();
     } catch (BadNumberFormatException e) {
       return 0;
     }
   }
+
+  // Verified double conversion.
+  static double _checkedToDouble(num value) {
+    if (value is !num) {
+      throw const IllegalArgumentException("num value expected");
+    }
+    return value.toDouble();
+  }
+  
   double operator +(num other) {
-    return add_(other.toDouble());
+    return add_(_checkedToDouble(other));
   }
   double add_(double other) native "Double_add";
 
   double operator -(num other) {
-    return sub_(other.toDouble());
+    return sub_(_checkedToDouble(other));
   }
   double sub_(double other) native "Double_sub";
 
   double operator *(num other) {
-    return mul_(other.toDouble());
+    return mul_(_checkedToDouble(other));
   }
   double mul_(double other) native "Double_mul";
 
   double operator ~/(num other) {
-    return trunc_div_(other.toDouble());
+    return trunc_div_(_checkedToDouble(other));
   }
   double trunc_div_(double other) native "Double_trunc_div";
 
   double operator /(num other) {
-    return div_(other.toDouble());
+    return div_(_checkedToDouble(other));
   }
   double div_(double other) native "Double_div";
 
   double operator %(num other) {
-    return modulo_(other.toDouble());
+    return modulo_(_checkedToDouble(other));
   }
   double modulo_(double other) native "Double_modulo";
 
   double remainder(num other) {
-    return remainder_(other.toDouble());
+    return remainder_(_checkedToDouble(other));
   }
   double remainder_(double other) native "Double_remainder";
 
   double operator negate() {
     return 0.0 - this;
   }
   bool operator ==(other) {
     if (!(other is num)) return false;
-    return equal_(other.toDouble());
+    return equal_(_checkedToDouble(other));
   }
   bool equal_(double other)native "Double_equal";
   bool equalToInteger(int other) native "Double_equalToInteger";
   bool operator <(num other) {
     return other > this;
   }
   bool operator >(num other) {
-    return greaterThan_(other.toDouble());
+    return greaterThan_(_checkedToDouble(other));
   }
   bool greaterThan_(double other) native "Double_greaterThan";
   bool operator >=(num other) {
     return (this == other) ||  (this > other);
   }
   bool operator <=(num other) {
     return (this == other) ||  (this < other);
   }
   double addFromInteger(int other) {
     return new Double.fromInteger(other) + this;
@@ skipping 117 matching lines @@
   }
   String toRadixString(int radix) {
     throw "Double.toRadixString unimplemented.";
   }
   int compareTo(Comparable other) {
     if (this == other) return 0;
     if (this < other) return -1;
     return 1;
   }
 }