Move a number of packages and some of pub over to using source_span.

pkg/yaml/lib/src/null_span.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 yaml.null_span;
 
-import 'package:path/path.dart' as p;
-import 'package:source_maps/source_maps.dart';
+import 'package:source_span/source_span.dart';
 
 /// A [Span] with no location information.
 ///
 /// This is used with [YamlMap.wrap] and [YamlList.wrap] to provide means of
 /// accessing a non-YAML map that behaves transparently like a map parsed from
 /// YAML.
-class NullSpan extends Span {
-  Location get end => start;
+class NullSpan extends SourceSpanMixin {
+  final SourceLocation start;
+  SourceLocation get end => start;
   final text = "";
 
-  NullSpan(String sourceUrl)
-      : this._(new NullLocation(sourceUrl));
-
-  NullSpan._(Location location)
-      : super(location, location, false);
-
-  String formatLocationMessage(String message, {bool useColors: false,
-      String color}) {
-    var locationMessage = sourceUrl == null ? "in an unknown location" :
-        "in ${p.prettyUri(sourceUrl)}";
-    return "$locationMessage: $message";
-  }
+  NullSpan(sourceUrl)
+      : start = new SourceLocation(0, sourceUrl: sourceUrl);
 }
-
-/// A [Location] with no location information.
-///
-/// This is used with [YamlMap.wrap] and [YamlList.wrap] to provide means of
-/// accessing a non-YAML map that behaves transparently like a map parsed from
-/// YAML.
-class NullLocation extends Location {
-  final String sourceUrl;
-  final line = 0;
-  final column = 0;
-
-  String get formatString => sourceUrl == null ? "unknown location" : sourceUrl;
-
-  NullLocation(this.sourceUrl)
-      : super(0);
-}