remove one more analyzer related TODO

lib/src/codegen/js_codegen.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.
 
 library dev_compiler.src.codegen.js_codegen;
 
 import 'dart:collection' show HashSet, HashMap;
 import 'dart:io' show Directory, File;
 
 import 'package:analyzer/analyzer.dart' hide ConstantEvaluator;
@@ skipping 2477 matching lines @@
       context.accept(visitor);
       return visitor._potentiallyMutated;
     }
     return true;
   }
   return false;
 }
 
 /// Adapted from VariableResolverVisitor. Finds an assignment to a given
 /// local variable.
-// TODO(jmesserly): change type annotation to not be *Impl once
-// isPotentiallyMutated is available on VariableElement.
 class _AssignmentFinder extends RecursiveAstVisitor {
   final VariableElement _variable;
   bool _potentiallyMutated = false;
 
   _AssignmentFinder(this._variable);
 
   @override
   visitSimpleIdentifier(SimpleIdentifier node) {
     // Ignore if qualified.
     AstNode parent = node.parent;
@@ skipping 12 matching lines @@
   }
 }
 
 // TODO(jmesserly): validate the library. See issue #135.
 bool _isJsNameAnnotation(DartObjectImpl value) => value.type.name == 'JsName';
 
 // TODO(jacobr): we would like to do something like the following
 // but we don't have summary support yet.
 // bool _supportJsExtensionMethod(AnnotatedNode node) =>
 //    _getAnnotation(node, "SupportJsExtensionMethod") != null;