some fixes for --strong warnings

lib/src/codegen/js_field_storage.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_field_storage;
 
 import 'dart:collection' show HashMap, HashSet;
 import 'package:analyzer/src/generated/ast.dart' show Identifier;
 import 'package:analyzer/src/generated/element.dart';
 
@@ skipping 38 matching lines @@
       found = true;
       // Record that the super property is overridden.
       if (superprop.library == library) overrides.add(superprop);
     }
   }
 
   // If this we found a super property, then this property overrides it.
   if (found) overrides.add(field);
 }
 
-PropertyInducingElement getProperty(
+FieldElement getProperty(
     ClassElement cls, LibraryElement fromLibrary, String name) {
   // Properties from a different library are not accessible.
   if (Identifier.isPrivateName(name) && cls.library != fromLibrary) {
     return null;
   }
   for (var accessor in cls.accessors) {
     var prop = accessor.variable;
     if (prop.name == name) return prop;
   }
   return null;
 }
 
 List<ClassElement> getSuperclasses(ClassElement cls) {
   var result = <ClassElement>[];
   var visited = new HashSet<ClassElement>();
   while (cls != null && visited.add(cls)) {
     for (var mixinType in cls.mixins.reversed) {
       var mixin = mixinType.element;
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }