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

tests/compiler/dart2js/simple_inferrer_test.dart

Index: tests/compiler/dart2js/simple_inferrer_test.dart
===================================================================
--- tests/compiler/dart2js/simple_inferrer_test.dart	(revision 31124)
+++ tests/compiler/dart2js/simple_inferrer_test.dart	(working copy)
@@ -538,6 +538,10 @@
   return a;
 }
 
+testReturnNotEquals() {
+  return new A() != 54;
+}
+
 testReturnInvokeDynamicGetter() => new A().myFactory();
 
 var topLevelConstList = const [42];
@@ -695,6 +699,7 @@
   testReturnNull4();
   testReturnNull5();
   testReturnNull6();
+  testReturnNotEquals();
 }
 """;
 
@@ -837,5 +842,6 @@
     checkReturn('testReturnNull4', typesTask.nullType);
     checkReturn('testReturnNull5', typesTask.nullType);
     checkReturn('testReturnNull6', typesTask.dynamicType);
+    checkReturn('testReturnNotEquals', typesTask.boolType);
   }));
 }