pkg/string_scanner: code format, expanded dependency constraint on unittest

lib/src/utils.dart

 // Copyright (c) 2014, 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.
 
 library string_scanner.utils;
 
 /// Validates the arguments passed to [StringScanner.error].
 void validateErrorArgs(String string, Match match, int position, int length) {
   if (match != null && (position != null || length != null)) {
     throw new ArgumentError("Can't pass both match and position/length.");
   }
 
   if (position != null) {
     if (position < 0) {
       throw new RangeError("position must be greater than or equal to 0.");
     } else if (position > string.length) {
       throw new RangeError("position must be less than or equal to the "
           "string length.");
     }
   }
 
   if (length != null && length < 0) {
     throw new RangeError("length must be greater than or equal to 0.");
   }
 
   if (position != null && length != null && position + length > string.length) {
     throw new RangeError("position plus length must not go beyond the end of "
         "the string.");
   }
-}
+}