Use MemberElement instead of Element in function sets and selectors.

pkg/compiler/lib/src/world.dart

Index: pkg/compiler/lib/src/world.dart
diff --git a/pkg/compiler/lib/src/world.dart b/pkg/compiler/lib/src/world.dart
index 686737b9688a3f46a66d8947bd6d9ff3f0a8bb19..109fbfa98c226a39c77605917a7e9b919eb29aa1 100644
--- a/pkg/compiler/lib/src/world.dart
+++ b/pkg/compiler/lib/src/world.dart
@@ -1068,7 +1068,7 @@ class ClosedWorldImpl implements ClosedWorld, ClosedWorldRefiner {
     // We're not tracking side effects of closures.
     if (selector.isClosureCall) return new SideEffects();
     SideEffects sideEffects = new SideEffects.empty();
-    for (Element e in allFunctions.filter(selector, mask)) {
+    for (MemberElement e in allFunctions.filter(selector, mask)) {
       if (e.isField) {
         if (selector.isGetter) {
           if (!fieldNeverChanges(e)) {