Only support the new source_span spans in barback.

pkg/barback/lib/src/span_wrapper.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_maps.span_wrapper;
-
-import 'package:source_maps/source_maps.dart';
-import 'package:source_span/source_span.dart' as source_span;
-
-/// A wrapper that exposes a [source_span.SourceSpan] as a [Span].
-class SpanWrapper extends Span {
-  final source_span.SourceSpan _inner;
-
-  String get text => _inner.text;
-
-  SpanWrapper(source_span.SourceSpan inner, bool isIdentifier)
-      : _inner = inner,
-        super(
-            new LocationWrapper(inner.start),
-            new LocationWrapper(inner.end),
-            isIdentifier);
-
-  static Span wrap(span, [bool isIdentifier = false]) {
-    if (span is Span) return span;
-    return new SpanWrapper(span, isIdentifier);
-  }
-}
-
-/// A wrapper that exposes a [source_span.SourceLocation] as a [Location].
-class LocationWrapper extends Location {
-  final source_span.SourceLocation _inner;
-
-  String get sourceUrl => _inner.sourceUrl.toString();
-  int get line => _inner.line;
-  int get column => _inner.column;
-
-  LocationWrapper(source_span.SourceLocation inner)
-      : _inner = inner,
-        super(inner.offset);
-
-  static Location wrap(location) {
-    if (location is Location) return location;
-    return new LocationWrapper(location);
-  }
-}