Add LineScanner and SpanScanner classes to string_scanner.

pkg/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.");
+  }
+}