Add SpanException and SpanFormatException classes to source_maps.

pkg/source_maps/lib/span.dart

 // Copyright (c) 2013, 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.
 
 /// Dart classes representing the souce spans and source files.
 library source_maps.span;
 
 import 'dart:math' show min, max;
 
 import 'package:path/path.dart' as p;
@@ skipping 342 matching lines @@
   int getOffset(int line, int column) =>
     super.getOffset(line - _baseLine,
         line == _baseLine ? column - _baseColumn : column) + _baseOffset;
 
   /// Retrieve the text associated with the specified range. This method
   /// operates on the real offsets from the original file, so that error
   /// messages can be reported accurately.
   String getText(int start, [int end]) =>
     super.getText(start - _baseOffset, end == null ? null : end - _baseOffset);
 }
+
+/// A class for exceptions that have source span information attached.
+class SpanException implements Exception {
+  /// A message describing the exception.
+  final String message;
+
+  /// The span associated with this exception.
+  ///
+  /// This may be `null` if the source location can't be determined.
+  final Span span;
+
+  SpanException(this.message, this.span);
+
+  String toString({bool useColors: false, String color}) {
+    if (span == null) return message;
+    return span.getLocationMessage(message, useColors: useColors, color: color);
+  }
+}
+
+/// A [SpanException] that's also a [FormatException].
+class SpanFormatException extends SpanException implements FormatException {
+  SpanFormatException(String message, Span span)
+      : super(message, span);
+}