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

lib/src/eager_span_scanner.dart

Index: lib/src/eager_span_scanner.dart
diff --git a/lib/src/eager_span_scanner.dart b/lib/src/eager_span_scanner.dart
index c537b0cd6f5ef5a3c0ad77fa308de53df007a8c3..f80dce5c3f9a4bdf39e8e624aff00ae30b6ad9c8 100644
--- a/lib/src/eager_span_scanner.dart
+++ b/lib/src/eager_span_scanner.dart
@@ -67,15 +67,26 @@ class EagerSpanScanner extends SpanScanner {
   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) {