Make the package strong-mode clean.

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 224 matching lines @@
     if (other is! _FileSpan) return super.compareTo(other);
 
     _FileSpan otherFile = other;
     var result = _start.compareTo(otherFile._start);
     return result == 0 ? _end.compareTo(otherFile._end) : result;
   }
 
   SourceSpan union(SourceSpan other) {
     if (other is! FileSpan) return super.union(other);
 
+    
     _FileSpan span = expand(other);
-    var beginSpan = span._start == _start ? this : other;
-    var endSpan = span._end == _end ? this : other;
 
-    if (beginSpan._end < endSpan._start) {
-      throw new ArgumentError("Spans $this and $other are disjoint.");
+    if (other is _FileSpan) {
+      if (this._start > other._end || other._start > this._end) {
+        throw new ArgumentError("Spans $this and $other are disjoint.");
+      }
+    } else {
+      if (this._start > other.end.offset || other.start.offset > this._end) {
+        throw new ArgumentError("Spans $this and $other are disjoint.");
+      }
     }
 
     return span;
   }
 
   bool operator ==(other) {
     if (other is! FileSpan) return super == other;
     if (other is! _FileSpan) {
       return super == other && sourceUrl == other.sourceUrl;
     }
@@ skipping 16 matching lines @@
       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);
     }
   }
 }