Address hint discovered by dart2js: add hashCode to source_maps.Span

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:utf' show stringToCodepoints;
 import 'dart:math' show min, max;
 
@@ skipping 51 matching lines @@
   String get formatLocation => start.formatString;
 
   String getLocationMessage(String message,
       {bool useColors: false, String color}) {
     return '$formatLocation: $message';
   }
 
   bool operator ==(Span other) =>
     sourceUrl == other.sourceUrl && start == other.start && end == other.end;
 
+  int get hashCode => sourceUrl.hashCode + start + (31 * (end - start));
+
   String toString() => '<$runtimeType: $start $end $formatLocation $text>';
 }
 
 /// A location in the source text
 abstract class Location implements Comparable {
   /// Url of the source containing this span.
   String get sourceUrl;
 
   /// The offset of this location, 0-based.
   final int offset;
@@ skipping 263 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);
 }