pkg/source_span: Eliminate dart2js warning about overriding `==`, but not `hashCode`.

lib/src/file.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 source_span.file;
 
 import 'dart:math' as math;
 import 'dart:typed_data';
 
 import 'location.dart';
@@ skipping 245 matching lines @@
   bool operator ==(other) {
     if (other is! FileSpan) return super == other;
     if (other is! _FileSpan) {
       return super == other && sourceUrl == other.sourceUrl;
     }
 
     return _start == other._start && _end == other._end &&
         sourceUrl == other.sourceUrl;
   }
 
+  // Eliminates dart2js warning about overriding `==`, but not `hashCode`
+  int get hashCode => super.hashCode;
+
   /// Returns a new span that covers both [this] and [other].
   ///
   /// Unlike [union], [other] may be disjoint from [this]. If it is, the text
   /// between the two will be covered by the returned span.
   FileSpan expand(FileSpan other) {
     if (sourceUrl != other.sourceUrl) {
       throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
           " \"${other.sourceUrl}\" don't match.");
     }
 
     if (other is _FileSpan) {
       var start = math.min(this._start, other._start);
       var end = math.max(this._end, other._end);
       return new _FileSpan(file, start, end);
     } else {
       var start = math.min(this._start, other.start.offset);
       var end = math.max(this._end, other.end.offset);
       return new _FileSpan(file, start, end);
     }
   }
 }