Report HintCode.MISSING_RETURN for async functions with return type that matters.

pkg/analyzer/lib/src/generated/resolver.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.resolver;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
@@ skipping 882 matching lines @@
     if (body is BlockFunctionBody) {
       // Generators are never required to have a return statement.
       if (body.isGenerator) {
         return;
       }
       // Check that the type is resolvable, and is not "void"
       DartType returnTypeType = returnType.type;
       if (returnTypeType == null || returnTypeType.isVoid) {
         return;
       }
-      // For async, give no hint if Future<Null> is assignable to the return
-      // type.
-      if (body.isAsynchronous &&
-          _typeSystem.isAssignableTo(_futureNullType, returnTypeType)) {
-        return;
+      // For async, give no hint if the return type does not matter, i.e.
+      // dynamic, Future<Null> or Future<dynamic>.
+      if (body.isAsynchronous) {
+        if (returnTypeType.isDynamic) {
+          return;
+        }
+        if (returnTypeType is InterfaceType &&
+            returnTypeType.isDartAsyncFuture) {
+          DartType futureArgument = returnTypeType.typeArguments[0];
+          if (futureArgument.isDynamic || futureArgument.isDartCoreNull) {
+            return;
+          }
+        }
       }
       // Check the block for a return statement, if not, create the hint
       if (!ExitDetector.exits(body)) {
         _errorReporter.reportErrorForNode(
             HintCode.MISSING_RETURN, returnType, [returnTypeType.displayName]);
       }
     }
   }
 
   /**
@@ skipping 9896 matching lines @@
       return null;
     }
     if (identical(node.staticElement, variable)) {
       if (node.inSetterContext()) {
         result = true;
       }
     }
     return null;
   }
 }