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

pkg/compiler/lib/src/js_backend/backend.dart

Index: pkg/compiler/lib/src/js_backend/backend.dart
diff --git a/pkg/compiler/lib/src/js_backend/backend.dart b/pkg/compiler/lib/src/js_backend/backend.dart
index 625abbda4c0747a1e571d4e6794fc8f003ea3ec9..e96fc421bdd9c571d528a9620c42fbc924dd2fa9 100644
--- a/pkg/compiler/lib/src/js_backend/backend.dart
+++ b/pkg/compiler/lib/src/js_backend/backend.dart
@@ -4,8 +4,6 @@
 
 library js_backend.backend;
 
-import 'dart:async' show Future;
-
 import 'package:js_runtime/shared/embedded_names.dart' as embeddedNames;
 
 import '../common.dart';
@@ -986,7 +984,7 @@ class JavaScriptBackend {
       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);