Add SpanScanner.within().

lib/src/span_scanner.dart

Index: lib/src/span_scanner.dart
diff --git a/lib/src/span_scanner.dart b/lib/src/span_scanner.dart
index dd16e47cf06c39f3233f1c11e4412bca96954844..dd7f0e4f2e261b99b5265c161bdfa8e6e94081ff 100644
--- a/lib/src/span_scanner.dart
+++ b/lib/src/span_scanner.dart
@@ -7,6 +7,7 @@ import 'package:source_span/source_span.dart';
 import 'eager_span_scanner.dart';
 import 'exception.dart';
 import 'line_scanner.dart';
+import 'relative_span_scanner.dart';
 import 'string_scanner.dart';
 import 'utils.dart';
 
@@ -69,6 +70,14 @@ class SpanScanner extends StringScanner implements LineScanner {
   factory SpanScanner.eager(String string, {sourceUrl, int position}) =
       EagerSpanScanner;
 
+  /// Creates a new [SpanScanner] that scans within [span].
+  ///
+  /// This scans through [span.text], but emits new spans from [span.file] in
+  /// their appropriate relative positions. The [string] field contains only
+  /// [span.text], and [position], [line], and [column] are all relative to the
+  /// span.
+  factory SpanScanner.within(FileSpan span) = RelativeSpanScanner;
+
   /// Creates a [FileSpan] representing the source range between [startState]
   /// and the current position.
   FileSpan spanFrom(LineScannerState startState, [LineScannerState endState]) {