Fix the typing of FileSpan.{start,end}.

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 228 matching lines @@
 /// Unlike the base [SourceSpan], [FileSpan] lazily computes its line and column
 /// values based on its offset and the contents of [file]. [FileSpan.message] is
 /// also able to provide more context then [SourceSpan.message], and
 /// [FileSpan.union] will return a [FileSpan] if possible.
 ///
 /// A [FileSpan] can be created using [SourceFile.span].
 abstract class FileSpan implements SourceSpanWithContext {
   /// The [file] that [this] belongs to.
   SourceFile get file;
 
+  FileLocation get start;
+  FileLocation get end;
+
   /// 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);
 }
 
 /// The implementation of [FileSpan].
 ///
 /// This is split into a separate class so that `is _FileSpan` checks can be run
@@ skipping 87 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);
     }
   }
 }