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

lib/src/string_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.
 
+import 'package:charcode/charcode.dart';
 import 'package:source_span/source_span.dart';
 
 import 'exception.dart';
 import 'utils.dart';
 
 /// When compiled to JS, forward slashes are always escaped in [RegExp.pattern].
 ///
 /// See issue 17998.
 final _slashAutoEscape = new RegExp("/").pattern == "\\/";
 
@@ skipping 57 matching lines @@
   ///
   /// This returns `null` if [offset] points outside the string. It doesn't
   /// affect [lastMatch].
   int peekChar([int offset]) {
     if (offset == null) offset = 0;
     var index = position + offset;
     if (index < 0 || index >= string.length) return null;
     return string.codeUnitAt(index);
   }
 
+  /// If the next character in the string is [character], consumes it.
+  ///
+  /// Returns whether or not [character] was consumed.
+  bool scanChar(int character) {
+    if (isDone) return false;
+    if (string.codeUnitAt(_position) != character) return false;
+    _position++;
+    return true;
+  }
+
+  /// If the next character in the string is [character], consumes it.
+  ///
+  /// If [character] could not be consumed, throws a [FormatException]
+  /// describing the position of the failure. [name] is used in this error as
+  /// the expected name of the character being matched; if it's `null`, the
+  /// character itself is used instead.
+  void expectChar(int character, {String name}) {
+    if (scanChar(character)) return;
+
+    if (name == null) {
+      if (character == $backslash) {
+        name = r'"\"';
+      } else if (character == $double_quote) {
+        name = r'"\""';
+      } else {
+        name = '"${new String.fromCharCode(character)}"';
+      }
+    }
+
+    _fail('Expected $name.');
+  }
+
   /// If [pattern] matches at the current position of the string, scans forward
   /// until the end of the match.
   ///
   /// Returns whether or not [pattern] matched.
   bool scan(Pattern pattern) {
     var success = matches(pattern);
     if (success) _position = _lastMatch.end;
     return success;
   }
 
@@ skipping 73 matching lines @@
     throw new StringScannerException(message, span, string);
   }
 
   // TODO(nweiz): Make this handle long lines more gracefully.
   /// Throws a [FormatException] describing that [name] is expected at the
   /// current position in the string.
   void _fail(String name) {
     error("expected $name.", position: this.position, length: 0);
   }
 }