Add StringScanner.scanChar() and .expectChar().

lib/src/eager_span_scanner.dart

 // Copyright (c) 2015, 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.
 
 import 'package:charcode/ascii.dart';
 
 import 'line_scanner.dart';
 import 'span_scanner.dart';
 
 // TODO(nweiz): Currently this duplicates code in line_scanner.dart. Once
@@ skipping 49 matching lines @@
       } else {
         _column = newPosition -
             string.lastIndexOf(_newlineRegExp, newPosition) - 1;
       }
     }
   }
 
   EagerSpanScanner(String string, {sourceUrl, int position})
       : super(string, sourceUrl: sourceUrl, position: position);
 
+  bool scanChar(int character) {
+    if (!super.scanChar(character)) return false;
+    _adjustLineAndColumn(character);
+    return true;
+  }
+
   int readChar() {
-    var char = super.readChar();
-    if (char == $lf || (char == $cr && peekChar() != $lf)) {
+    var character = super.readChar();
+    _adjustLineAndColumn(character);
+    return character;
+  }
+
+  /// Adjusts [_line] and [_column] after having consumed [character].
+  void _adjustLineAndColumn(int character) {
+    if (character == $lf || (character == $cr && peekChar() != $lf)) {
       _line += 1;
       _column = 0;
     } else {
       _column += 1;
     }
-    return char;
   }
 
   bool scan(Pattern pattern) {
     if (!super.scan(pattern)) return false;
 
     var newlines = _newlinesIn(lastMatch[0]);
     _line += newlines.length;
     if (newlines.isEmpty) {
       _column += lastMatch[0].length;
     } else {
@@ skipping 15 matching lines @@
 /// A class representing the state of an [EagerSpanScanner].
 class _EagerSpanScannerState implements LineScannerState {
   final EagerSpanScanner _scanner;
   final int position;
   final int line;
   final int column;
 
   _EagerSpanScannerState(this._scanner, this.position, this.line, this.column);
 }