Remove support for the old Span classes from source_maps.

pkg/source_maps/lib/source_maps.dart

 // Copyright (c) 2013, 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 to create and parse source maps.
 ///
 /// Create a source map using [SourceMapBuilder]. For example:
 ///     var json = (new SourceMapBuilder()
 ///         ..add(inputSpan1, outputSpan1)
 ///         ..add(inputSpan2, outputSpan2)
 ///         ..add(inputSpan3, outputSpan3)
 ///         .toJson(outputFile);
 ///
-/// Use the [Span] and [SourceFile] classes to specify span locations.
+/// Use the source_span package's [SourceSpan] and [SourceFile] classes to
+/// specify span locations.
 ///
 /// Parse a source map using [parse], and call `spanFor` on the returned mapping
 /// object. For example:
 ///     var mapping = parse(json);
 ///     mapping.spanFor(outputSpan1.line, outputSpan1.column)
 ///
 /// ## Getting the code ##
 ///
 /// This library is distributed as a [pub][] package. To install this package,
 /// add the following to your `pubspec.yaml` file:
 ///
 ///     dependencies:
 ///       source_maps: any
 ///
 /// After you run `pub install`, you should be able to access this library by
 /// importing `package:source_maps/source_maps.dart`.
 ///
 /// For more information, see the
 /// [source_maps package on pub.dartlang.org][pkg].
 ///
 /// [pub]: http://pub.dartlang.org
 /// [pkg]: http://pub.dartlang.org/packages/source_maps
 library source_maps;
 
 export "builder.dart";
 export "parser.dart";
 export "printer.dart";
 export "refactor.dart";
-export "span.dart";
+export 'src/source_map_span.dart';