Remove support for the old Span classes from source_maps.

pkg/source_maps/lib/source_maps.dart

Index: pkg/source_maps/lib/source_maps.dart
diff --git a/pkg/source_maps/lib/source_maps.dart b/pkg/source_maps/lib/source_maps.dart
index 2d4a4ccc42bc6ee69dc85bd85153af464cd077df..95319034111f615fdbcf022ca1ab5db26a91a489 100644
--- a/pkg/source_maps/lib/source_maps.dart
+++ b/pkg/source_maps/lib/source_maps.dart
@@ -11,7 +11,8 @@
 ///         ..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:
@@ -40,4 +41,4 @@ export "builder.dart";
 export "parser.dart";
 export "printer.dart";
 export "refactor.dart";
-export "span.dart";
+export 'src/source_map_span.dart';