Add documentation for StringToken.stringValue

pkg/compiler/lib/src/scanner/token.dart

Index: pkg/compiler/lib/src/scanner/token.dart
diff --git a/pkg/compiler/lib/src/scanner/token.dart b/pkg/compiler/lib/src/scanner/token.dart
index c7c74d1b4a39e126b61204d08da0b9942bcf4314..2dff11e07a4b00e82d1a16e6f9c771e607a89bfe 100644
--- a/pkg/compiler/lib/src/scanner/token.dart
+++ b/pkg/compiler/lib/src/scanner/token.dart
@@ -125,7 +125,7 @@ abstract class Token implements Spannable {
    * produces ..., SymbolToken($), StringToken(a), StringToken((), ...
    *
    * After parsing the identifier 'a', the parser tests for a function
-   * declaration using [:identical(next.stringValue, '('):], which (rihgtfully)
+   * declaration using [:identical(next.stringValue, '('):], which (rightfully)
    * returns false because stringValue returns [:null:].
    */
   String get stringValue;
@@ -368,6 +368,7 @@ class StringToken extends Token {
     }
   }
 
+  /// See [Token.stringValue] for an explanation.
   String get stringValue => null;
 
   bool isIdentifier() => identical(kind, IDENTIFIER_TOKEN);