Infer Null for return type of functions with empty returns.

pkg/analyzer/lib/src/generated/testing/test_type_provider.dart

Index: pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
diff --git a/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart b/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
index 8460484fd2dceeaaba9b7b6f6b7b2a3846d5c1db..cb70a5333183e6f887d0a72c26e7c67546f1b6c5 100644
--- a/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
+++ b/pkg/analyzer/lib/src/generated/testing/test_type_provider.dart
@@ -66,6 +66,11 @@ class TestTypeProvider extends TypeProviderBase {
   /**
    * The type representing the built-in type 'FutureOr'
    */
+  InterfaceType _futureOrNullType;
+
+  /**
+   * The type representing the built-in type 'FutureOr'
+   */
   InterfaceType _futureOrType;
 
   /**
@@ -267,6 +272,14 @@ class TestTypeProvider extends TypeProviderBase {
   }
 
   @override
+  InterfaceType get futureOrNullType {
+    if (_futureOrNullType == null) {
+      _futureOrNullType = futureOrType.instantiate(<DartType>[nullType]);
+    }
+    return _futureOrNullType;
+  }
+
+  @override
   InterfaceType get futureOrType {
     if (_futureOrType == null) {
       Source asyncSource = _context.sourceFactory.forUri(DartSdk.DART_ASYNC);