Avoid instantiating FileLocations where possible in source_span.

pkg/source_span/lib/src/span_mixin.dart

Index: pkg/source_span/lib/src/span_mixin.dart
diff --git a/pkg/source_span/lib/src/span_mixin.dart b/pkg/source_span/lib/src/span_mixin.dart
index 95a720aaef3a565ddaa22b31e463f87bdcaca2a4..dbfbcadd4dc9dd1644dc337614b591afd29732ac 100644
--- a/pkg/source_span/lib/src/span_mixin.dart
+++ b/pkg/source_span/lib/src/span_mixin.dart
@@ -65,7 +65,7 @@ abstract class SourceSpanMixin implements SourceSpan {
     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);