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

pkg/fixnum/lib/src/int64.dart

Index: pkg/fixnum/lib/src/int64.dart
diff --git a/pkg/fixnum/lib/src/int64.dart b/pkg/fixnum/lib/src/int64.dart
index f12631c5288b69643c73cc04aaf71e85cb674cd4..192b11f68a89ab0aad10a0b013b4baeca0dbe39f 100644
--- a/pkg/fixnum/lib/src/int64.dart
+++ b/pkg/fixnum/lib/src/int64.dart
@@ -81,7 +81,7 @@ class Int64 implements IntX {
       int c = s.codeUnitAt(i);
       int digit = Int32._decodeDigit(c);
       if (digit < 0 || digit >= radix) {
-        throw new Exception("Non-radix char code: $c");
+        throw new FormatException("Non-radix char code: $c");
       }
 
       // [radix] and [digit] are at most 6 bits, component is 22, so we can