!= is a shortcut to == true, so it returns a boolean.

tests/compiler/dart2js/simple_inferrer_test.dart

 // Copyright (c) 2013, 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.
 
 import 'package:expect/expect.dart';
 import "package:async_helper/async_helper.dart";
 import
     '../../../sdk/lib/_internal/compiler/implementation/types/types.dart'
     show TypeMask;
 
@@ skipping 520 matching lines @@
   if (a != null) return null;
   return a;
 }
 
 testReturnNull6() {
   var a = topLeveGetter();
   if (a == null) return 42;
   return a;
 }
 
+testReturnNotEquals() {
+  return new A() != 54;
+}
+
 testReturnInvokeDynamicGetter() => new A().myFactory();
 
 var topLevelConstList = const [42];
 
 get topLevelGetter => 42;
 returnDynamic() => topLevelGetter(42);
 returnTopLevelGetter() => topLevelGetter;
 
 class A {
   factory A() = A.generative;
@@ skipping 137 matching lines @@
   testCascade2();
   testSpecialization1();
   testSpecialization2();
   testSpecialization3();
   testReturnNull1(topLevelGetter());
   testReturnNull2(topLevelGetter());
   testReturnNull3(topLevelGetter());
   testReturnNull4();
   testReturnNull5();
   testReturnNull6();
+  testReturnNotEquals();
 }
 """;
 
 void main() {
   Uri uri = new Uri(scheme: 'source');
   var compiler = compilerFor(TEST, uri);
   asyncTest(() => compiler.runCompiler(uri).then((_) {
     var typesTask = compiler.typesTask;
     var typesInferrer = typesTask.typesInferrer;
 
@@ skipping 122 matching lines @@
         findElement(compiler, 'CascadeHelper')));
     checkReturn('testSpecialization1', typesTask.numType);
     checkReturn('testSpecialization2', typesTask.dynamicType);
     checkReturn('testSpecialization3', typesTask.uint31Type.nullable());
     checkReturn('testReturnNull1', typesTask.nullType);
     checkReturn('testReturnNull2', typesTask.nullType);
     checkReturn('testReturnNull3', typesTask.dynamicType);
     checkReturn('testReturnNull4', typesTask.nullType);
     checkReturn('testReturnNull5', typesTask.nullType);
     checkReturn('testReturnNull6', typesTask.dynamicType);
+    checkReturn('testReturnNotEquals', typesTask.boolType);
   }));
 }