Bug cleanup: unify on allowing radices from 2 to 36 in

runtime/lib/integers.dart

Index: runtime/lib/integers.dart
diff --git a/runtime/lib/integers.dart b/runtime/lib/integers.dart
index 09a3f689944c5aa50f5b775696db928a8cf392e3..14ed12301620e6b1229ed343d5c6c1193ed97753 100644
--- a/runtime/lib/integers.dart
+++ b/runtime/lib/integers.dart
@@ -154,9 +154,11 @@ class _IntegerImplementation {
   }
   String toRadixString(int radix) {
     final table = const ["0", "1", "2", "3", "4", "5", "6", "7", "8", "9",
-                         "a", "b", "c", "d", "e", "f"];
-    if ((radix <= 1) || (radix > 16)) {
-      throw "Bad radix: $radix";
+                         "a", "b", "c", "d", "e", "f", "g", "h", "i", "j",
+                         "k", "l", "m", "n", "o", "p", "q", "r", "s", "t",
+                         "u", "v", "w", "x", "y", "z"];
+    if (radix < 2 || radix > 36) {
+      throw new IllegalArgumentException(radix);
     }
     final bool isNegative = this < 0;
     var value = isNegative ? -this : this;