Use MemberEntity more in type_graph_nodes

pkg/compiler/lib/src/inferrer/inferrer_engine.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_engine.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_engine.dart b/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
index 21d73f961270c5ea6e812180f7715fd0ed76c7e2..cb7475b1a400e4e4d8ae713eaa0daaddcb545c1e 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_engine.dart
@@ -1044,7 +1044,7 @@ class InferrerEngine {
     types.allocatedLists.values.forEach(cleanup);
   }
 
-  Iterable<Element> getCallersOf(MemberElement element) {
+  Iterable<MemberEntity> getCallersOf(MemberElement element) {
     if (compiler.disableTypeInference) {
       throw new UnsupportedError(
           "Cannot query the type inferrer when type inference is disabled.");