Don't crash on deserialized type inference.

pkg/compiler/lib/src/serialization/task.dart

Index: pkg/compiler/lib/src/serialization/task.dart
diff --git a/pkg/compiler/lib/src/serialization/task.dart b/pkg/compiler/lib/src/serialization/task.dart
index 0eb9db08ffbcd538900ea3fb40e8fee5be006064..1639c323667d9875b4ad7d7cfb3cc68c0c0516c4 100644
--- a/pkg/compiler/lib/src/serialization/task.dart
+++ b/pkg/compiler/lib/src/serialization/task.dart
@@ -65,11 +65,11 @@ class SerializationTask extends CompilerTask implements LibraryDeserializer {
         element, context, deserializer.computeWorldImpact(element));
   }
 
-  bool hasResolvedAst(Element element) {
+  bool hasResolvedAst(ExecutableElement element) {
     return deserializer != null ? deserializer.hasResolvedAst(element) : false;
   }
 
-  ResolvedAst getResolvedAst(Element element) {
+  ResolvedAst getResolvedAst(ExecutableElement element) {
     return deserializer != null ? deserializer.getResolvedAst(element) : null;
   }
 }
@@ -102,8 +102,8 @@ class DeserializedResolutionWorkItem implements ResolutionWorkItem {
 abstract class DeserializerSystem {
   Future<LibraryElement> readLibrary(Uri resolvedUri);
   bool isDeserialized(Element element);
-  bool hasResolvedAst(Element element);
-  ResolvedAst getResolvedAst(Element element);
+  bool hasResolvedAst(ExecutableElement element);
+  ResolvedAst getResolvedAst(ExecutableElement element);
   bool hasResolutionImpact(Element element);
   ResolutionImpact getResolutionImpact(Element element);
   WorldImpact computeWorldImpact(Element element);