Add an additional optional argument to SpanScanner.spanFrom.

pkg/string_scanner/lib/src/span_scanner.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 string_scanner.span_scanner;
 
 import 'package:source_span/source_span.dart';
 
 import 'exception.dart';
 import 'line_scanner.dart';
@@ skipping 40 matching lines @@
   ///
   /// [sourceUrl] is used as [SourceLocation.sourceUrl] for the returned
   /// [FileSpan]s as well as for error reporting. It can be a [String], a
   /// [Uri], or `null`.
   SpanScanner(String string, {sourceUrl, int position})
       : _sourceFile = new SourceFile(string, url: sourceUrl),
         super(string, sourceUrl: sourceUrl, position: position);
 
   /// Creates a [FileSpan] representing the source range between [startState]
   /// and the current position.
-  FileSpan spanFrom(LineScannerState startState) =>
-      _sourceFile.span(startState.position, position);
+  FileSpan spanFrom(LineScannerState startState, [LineScannerState endState]) {
+    var endPosition = endState == null ? position : endState.position;
+    return _sourceFile.span(startState.position, endPosition);
+  }
 
   bool matches(Pattern pattern) {
     if (!super.matches(pattern)) {
       _lastSpan = null;
       return false;
     }
 
     _lastSpan = _sourceFile.span(position, lastMatch.end);
     return true;
   }
@@ skipping 16 matching lines @@
 class _SpanScannerState implements LineScannerState {
   /// The [SpanScanner] that created this.
   final SpanScanner _scanner;
 
   final int position;
   int get line => _scanner._sourceFile.getLine(position);
   int get column => _scanner._sourceFile.getColumn(position);
 
   _SpanScannerState(this._scanner, this.position);
 }