Avoid instantiating FileLocations where possible in source_span.

pkg/source_span/lib/src/span_mixin.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.span_mixin;
 
 import 'package:path/path.dart' as p;
 
 import 'colors.dart' as colors;
 import 'span.dart';
@@ skipping 47 matching lines @@
     buffer.write("\n");
     var textLine = text.split("\n").first;
     if (color != null) buffer.write(color);
     buffer.write(textLine);
     buffer.write("\n");
     buffer.write('^' * textLine.length);
     if (color != null) buffer.write(colors.NONE);
     return buffer.toString();
   }
 
-  bool operator ==(SourceSpan other) =>
+  bool operator ==(other) => other is SourceSpan &&
       start == other.start && end == other.end;
 
   int get hashCode => start.hashCode + (31 * end.hashCode);
 
   String toString() => '<$runtimeType: from $start to $end "$text">';
 }