Fix dart2js warnings and add test to ensure it stays clean.

pkg/compiler/lib/src/elements/elements.dart

 // Copyright (c) 2012, 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 elements;
 
 import 'package:front_end/src/fasta/parser/async_modifier.dart'
     show AsyncModifier;
 
 import '../common.dart';
@@ skipping 452 matching lines @@
         element.enclosingElement != null &&
         (element.enclosingElement.kind == ElementKind.CLASS ||
             element.enclosingElement.kind == ElementKind.COMPILATION_UNIT ||
             element.enclosingElement.kind == ElementKind.LIBRARY ||
             element.enclosingElement.kind == ElementKind.PREFIX);
   }
 
   static bool isInStaticContext(Element element) {
     if (isUnresolved(element)) return true;
     if (element.enclosingElement.isClosure) {
-      var closureClass = element.enclosingElement;
+      dynamic closureClass = element.enclosingElement;
+      // ignore: UNDEFINED_GETTER
       element = closureClass.methodElement;
     }
     Element outer = element.outermostEnclosingMemberOrTopLevel;
     if (isUnresolved(outer)) return true;
     if (outer.isTopLevel) return true;
     if (outer.isGenerativeConstructor) return false;
     if (outer.isInstanceMember) return false;
     return true;
   }
 
@@ skipping 1543 matching lines @@
   /// by a field.
   bool get isDeclaredByField;
 
   /// Returns `true` if this member is abstract.
   bool get isAbstract;
 
   /// If abstract, [implementation] points to the overridden concrete member,
   /// if any. Otherwise [implementation] points to the member itself.
   Member get implementation;
 }