Use MemberElement instead of Element in function sets and selectors.

pkg/compiler/lib/src/types/flat_type_mask.dart

Index: pkg/compiler/lib/src/types/flat_type_mask.dart
diff --git a/pkg/compiler/lib/src/types/flat_type_mask.dart b/pkg/compiler/lib/src/types/flat_type_mask.dart
index cbeba31a3e638009c350e4be7816517b0450e40b..15caf644d2bd4059f60c6bf41211d67111bf0a0c 100644
--- a/pkg/compiler/lib/src/types/flat_type_mask.dart
+++ b/pkg/compiler/lib/src/types/flat_type_mask.dart
@@ -569,7 +569,8 @@ class FlatTypeMask implements TypeMask {
   MemberElement locateSingleElement(
       Selector selector, ClosedWorld closedWorld) {
     if (isEmptyOrNull) return null;
-    Iterable<Element> targets = closedWorld.allFunctions.filter(selector, this);
+    Iterable<MemberElement> targets =
+        closedWorld.allFunctions.filter(selector, this);
     if (targets.length != 1) return null;
     Element result = targets.first;
     ClassElement enclosing = result.enclosingClass.declaration;