Fix issue 100:

lib/double.dart

Index: lib/double.dart
===================================================================
--- lib/double.dart	(revision 394)
+++ lib/double.dart	(working copy)
@@ -12,38 +12,47 @@
       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";
 
@@ -52,7 +61,7 @@
   }
   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";
@@ -60,7 +69,7 @@
     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) {