Roll Observatory packages and add a roll script

string_scanner/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.");
-  }
-}