Infer Null for return type of functions with empty returns.

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 2288 matching lines @@
         }
         if (expectedReturnType is InterfaceType &&
             expectedReturnType.isDartAsyncFuture) {
           DartType futureArgument = expectedReturnType.typeArguments[0];
           if (futureArgument.isDynamic ||
               futureArgument.isDartCoreNull ||
               futureArgument.isObject) {
             return;
           }
         }
-      } else if (expectedReturnType.isDynamic || expectedReturnType.isVoid) {
+      } else if (expectedReturnType.isDynamic ||
+          expectedReturnType.isVoid ||
+          _typeSystem.isSubtypeOf(
+              expectedReturnType, _typeProvider.futureOrNullType)) {

[Jennifer Messerly, 2017/02/03 18:47:53]

If I have:

    Future<Null> foo() {
      return;
    }

shouldn't that be a RETURN_WITHOUT_VALUE?

[Leaf, 2017/02/06 18:27:15]

Good catch, thanks!  Fixed to keep it simple for now.

         return;
       }
       _hasReturnWithoutValue = true;
       _errorReporter.reportErrorForNode(
           StaticWarningCode.RETURN_WITHOUT_VALUE, statement);
       return;
     } else if (_inGenerator) {
       // RETURN_IN_GENERATOR
       _errorReporter.reportErrorForNode(
           CompileTimeErrorCode.RETURN_IN_GENERATOR,
@@ skipping 4757 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }