Fix return type checking for generator functions.

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 engine.resolver;
 
 import "dart:math" as math;
 import 'dart:collection';
 
 import 'package:analyzer/src/generated/utilities_collection.dart';
@@ skipping 564 matching lines @@
    */
   bool _checkForMissingReturn(TypeName returnType, FunctionBody body) {
     // Check that the method or function has a return type, and a function body
     if (returnType == null || body == null) {
       return false;
     }
     // Check that the body is a BlockFunctionBody
     if (body is! BlockFunctionBody) {
       return false;
     }
+    // Generators are never required to have a return statement.
+    if (body.isGenerator) {
+      return false;
+    }
     // Check that the type is resolvable, and is not "void"
     DartType returnTypeType = returnType.type;
     if (returnTypeType == null || returnTypeType.isVoid) {
       return false;
     }
     // For async, give no hint if Future<Null> is assignable to the return
     // type.
     if (body.isAsynchronous && _futureNullType.isAssignableTo(returnTypeType)) {
       return false;
     }
@@ skipping 15002 matching lines @@
    * library.
    */
   final HashSet<String> members = new HashSet<String>();
 
   /**
    * Names of resolved or unresolved class members that are read in the
    * library.
    */
   final HashSet<String> readMembers = new HashSet<String>();
 }