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

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

Index: pkg/compiler/lib/src/elements/elements.dart
diff --git a/pkg/compiler/lib/src/elements/elements.dart b/pkg/compiler/lib/src/elements/elements.dart
index 3315cb638bc3dbc09584ad1283d7af7ed767a748..1b2e6a58404ed0b5423c3ca3fca59608aab81338 100644
--- a/pkg/compiler/lib/src/elements/elements.dart
+++ b/pkg/compiler/lib/src/elements/elements.dart
@@ -470,7 +470,8 @@ class Elements {
   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;