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';
 import 'utils.dart';
 
 /// A mixin for easily implementing [SourceSpan].
 ///
 /// This implements the [SourceSpan] methods in terms of [start], [end], and
 /// [text]. This assumes that [start] and [end] have the same source URL, that
 /// [start] comes before [end], and that [text] has a number of characters equal
 /// to the distance between [start] and [end].
 abstract class SourceSpanMixin implements SourceSpan {
   Uri get sourceUrl => start.sourceUrl;
   int get length => end.offset - start.offset;
 
   int compareTo(SourceSpan other) {
-    int d = start.compareTo(other.start);
-    return d == 0 ? end.compareTo(other.end) : d;
+    var result = start.compareTo(other.start);
+    return result == 0 ? end.compareTo(other.end) : result;
   }
 
   SourceSpan union(SourceSpan other) {
     if (sourceUrl != other.sourceUrl) {
       throw new ArgumentError("Source URLs \"${sourceUrl}\" and "
           " \"${other.sourceUrl}\" don't match.");
     }
 
     var start = min(this.start, other.start);
     var end = max(this.end, other.end);
@@ skipping 22 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">';
 }