Add LineScanner and SpanScanner classes to string_scanner.

pkg/string_scanner/lib/src/exception.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.exception;
+
+import 'package:source_maps/source_maps.dart';
+
+/// An exception thrown by a [StringScanner] that failed to parse a string.
+class StringScannerException implements FormatException {
+  /// The error message.
+  final String message;
+
+  /// The source string being parsed.
+  final String string;
+
+  /// The URL of the source file being parsed.
+  ///
+  /// This may be `null`, indicating that the source URL is unknown.
+  final Uri sourceUrl;
+
+  /// The span within [string] that caused the exception.
+  final Span span;
+
+  StringScannerException(this.message, this.string, this.sourceUrl, this.span);
+
+  /// Returns a detailed description of this exception.
+  ///
+  /// If [useColors] is true, the section of the source that caused the
+  /// exception will be colored using ANSI color codes. By default, it's colored
+  /// red, but a different ANSI code may passed via [color].
+  String toString({bool useColors: false, String color}) {
+    return "Error on " + span.getLocationMessage(
+        message, useColors: useColors, color: color);
+  }
+}
+