Add StringLengthConstantExpression

pkg/docgen/lib/src/models/model_helpers.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 docgen.model_helpers;
 
 import 'dart:collection';
 
 import 'package:compiler/src/constants/expressions.dart';
 
@@ skipping 211 matching lines @@
   }
 
   @override
   Annotation visitStringFromEnvironment(
       StringFromEnvironmentConstantExpression exp,
       AnnotationInfo context) {
     return null;
   }
 
   @override
+  Annotation visitStringLength(StringLengthConstantExpression exp,
+                               AnnotationInfo context) {
+    return null;
+  }
+
+  @override
   Annotation visitDeferred(DeferredConstantExpression exp,
                            AnnotationInfo context) {
     return exp.expression.accept(this, context);
   }
 }
 
 /// A declaration is private if itself is private, or the owner is private.
 bool isHidden(DeclarationSourceMirror mirror) {
   if (mirror is LibraryMirror) {
     return _isLibraryPrivate(mirror);
@@ skipping 186 matching lines @@
   // the time.
   var sdkLibrary = LIBRARIES[dart2js_util.nameOf(mirror)];
   if (sdkLibrary != null) {
     return !sdkLibrary.documented;
   } else if (dart2js_util.nameOf(mirror).startsWith('_') || dart2js_util.nameOf(
       mirror).contains('._')) {
     return true;
   }
   return false;
 }