Restore weak mode errors on empty returns in Null functions

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 2299 matching lines @@
             expectedReturnType.isDartAsyncFuture) {
           DartType futureArgument = expectedReturnType.typeArguments[0];
           if (futureArgument.isDynamic ||
               futureArgument.isDartCoreNull ||
               futureArgument.isObject) {
             return;
           }
         }
       } else if (expectedReturnType.isDynamic ||
           expectedReturnType.isVoid ||
-          expectedReturnType.isDartCoreNull) {
+          (expectedReturnType.isDartCoreNull && _options.strongMode)) {
+        // TODO(leafp): Empty returns shouldn't be allowed for Null in strong
+        // mode either once we allow void as a type argument.  But for now, the
+        // only type we can validly infer for f.then((_) {print("hello");}) is
+        // Future<Null>, so we allow this.
         return;
       }
       _hasReturnWithoutValue = true;
       _errorReporter.reportErrorForNode(
           StaticWarningCode.RETURN_WITHOUT_VALUE, statement);
       return;
     } else if (_inGenerator) {
       // RETURN_IN_GENERATOR
       _errorReporter.reportErrorForNode(
           CompileTimeErrorCode.RETURN_IN_GENERATOR,
@@ skipping 4759 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);
     }
   }
 }