Throw FormatException when Int32/Int64.parseRadix() encounters bad digit

pkg/fixnum/lib/src/int32.dart

Index: pkg/fixnum/lib/src/int32.dart
diff --git a/pkg/fixnum/lib/src/int32.dart b/pkg/fixnum/lib/src/int32.dart
index c013f916085adae32fbb6d6f0a0ba50f615e36fd..d7000401d855008432763a2ec0d8e71c1580e9ab 100644
--- a/pkg/fixnum/lib/src/int32.dart
+++ b/pkg/fixnum/lib/src/int32.dart
@@ -71,7 +71,7 @@ class Int32 implements IntX {
       int c = s.codeUnitAt(i);
       int digit = _decodeDigit(c);
       if (digit < 0 || digit >= radix) {
-        throw new Exception("Non-radix code unit: $c");
+        throw new FormatException("Non-radix code unit: $c");
       }
       x = (x * radix) + digit;
     }