Add SourceLocationMixin and SourceLocationBase.

lib/src/location.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.location;
 
 import 'span.dart';
 
-// A class that describes a single location within a source file.
+// TODO(nweiz): Use SourceLocationMixin once we decide to cut a release with
+// breaking changes. See SourceLocationMixin for details.
+
+/// A class that describes a single location within a source file.
+///
+/// This class should not be extended. Instead, [SourceLocationBase] should be
+/// extended instead.
 class SourceLocation implements Comparable<SourceLocation> {
   /// URL of the source containing this location.
   ///
   /// This may be null, indicating that the source URL is unknown or
   /// unavailable.
   final Uri sourceUrl;
 
   /// The 0-based offset of this location in the source.
   final int offset;
 
@@ skipping 58 matching lines @@
   }
 
   bool operator ==(other) =>
       other is SourceLocation && sourceUrl == other.sourceUrl &&
       offset == other.offset;
 
   int get hashCode => sourceUrl.hashCode + offset;
 
   String toString() => '<$runtimeType: $offset $toolString>';
 }
+
+/// A base class for source locations with [offset], [line], and [column] known
+/// at construction time.
+class SourceLocationBase extends SourceLocation {
+  SourceLocationBase(int offset, {sourceUrl, int line, int column})
+      : super(offset, sourceUrl: sourceUrl, line: line, column: column);
+}