Infer Null for return type of functions with empty returns.

pkg/analyzer/lib/src/summary/summary_sdk.dart

Index: pkg/analyzer/lib/src/summary/summary_sdk.dart
diff --git a/pkg/analyzer/lib/src/summary/summary_sdk.dart b/pkg/analyzer/lib/src/summary/summary_sdk.dart
index 0a618061912af23d96e1c2a3ed4885445923d8a2..5ac11b73173915768f03ec0cc77859b293df86df 100644
--- a/pkg/analyzer/lib/src/summary/summary_sdk.dart
+++ b/pkg/analyzer/lib/src/summary/summary_sdk.dart
@@ -123,6 +123,7 @@ class SummaryTypeProvider extends TypeProviderBase {
   InterfaceType _functionType;
   InterfaceType _futureDynamicType;
   InterfaceType _futureNullType;
+  InterfaceType _futureOrNullType;
   InterfaceType _futureOrType;
   InterfaceType _futureType;
   InterfaceType _intType;
@@ -190,6 +191,13 @@ class SummaryTypeProvider extends TypeProviderBase {
   }
 
   @override
+  InterfaceType get futureOrNullType {
+    assert(_asyncLibrary != null);
+    _futureOrNullType ??= futureOrType.instantiate(<DartType>[nullType]);
+    return _futureOrNullType;
+  }
+
+  @override
   InterfaceType get futureOrType {
     assert(_asyncLibrary != null);
     try {