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

pkg/compiler/lib/src/js_backend/backend.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 js_backend.backend;
 
-import 'dart:async' show Future;
-
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
 import '../common.dart';
 import '../common/backend_api.dart'
     show BackendClasses, ForeignResolver, NativeRegistry, ImpactTransformer;
 import '../common/codegen.dart' show CodegenImpact, CodegenWorkItem;
 import '../common/names.dart' show Uris;
 import '../common/resolution.dart'
     show Frontend, Resolution, ResolutionImpact, Target;
 import '../common/tasks.dart' show CompilerTask;
@@ skipping 960 matching lines @@
     var kind = element.kind;
     if (kind == ElementKind.TYPEDEF) {
       return const WorldImpact();
     }
     if (element.isConstructor &&
         element.enclosingClass == helpers.jsNullClass) {
       // Work around a problem compiling JSNull's constructor.
       return const CodegenImpact();
     }
     if (kind.category == ElementCategory.VARIABLE) {
-      VariableElement variableElement = element;
+      VariableElement variableElement = element as VariableElement;

[Johnni Winther, 2017/04/03 12:07:33]

Why is this necessary? VariableElement is a subtype of Element.

[ahe, 2017/04/03 12:25:07]

Done.

       ConstantExpression constant = variableElement.constant;
       if (constant != null) {
         ConstantValue initialValue = constants.getConstantValue(constant);
         if (initialValue != null) {
           work.registry.worldImpact
               .registerConstantUse(new ConstantUse.init(initialValue));
           // We don't need to generate code for static or top-level
           // variables. For instance variables, we may need to generate
           // the checked setter.
           if (Elements.isStaticOrTopLevel(element)) {
@@ skipping 608 matching lines @@
 
   bool canUseAliasedSuperMember(MemberEntity member, Selector selector) {
     return !selector.isGetter;
   }
 
   /// Returns `true` if [member] is called from a subclass via `super`.
   bool isAliasedSuperMember(MemberEntity member) {
     return _aliasedSuperMembers.contains(member);
   }
 }