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

sdk/lib/_internal/compiler/implementation/inferrer/inferrer_visitor.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.
 
 library inferrer_visitor;
 
 import '../dart2jslib.dart' hide Selector, TypedSelector;
 import '../dart_types.dart';
 import '../elements/elements.dart';
 import '../tree/tree.dart';
@@ skipping 829 matching lines @@
       T receiverType = visit(node.receiver);
       DartType type = elements.getType(node.arguments.head);
       return types.narrowType(receiverType, type);
     } else if (node.argumentsNode is Prefix) {
       // Unary operator.
       return visitDynamicSend(node);
     } else if ('===' == op.source
                || '!==' == op.source) {
       node.visitChildren(this);
       return types.boolType;
+    } else if ('!=' == op.source) {
+      visitDynamicSend(node);
+      return types.boolType;
     } else {
       // Binary operator.
       return visitDynamicSend(node);
     }
   }
 
   // Because some nodes just visit their children, we may end up
   // visiting a type annotation, that may contain a send in case of a
   // prefixed type. Therefore we explicitly visit the type annotation
   // to avoid confusing the [ResolvedVisitor].
@@ skipping 291 matching lines @@
     return type;
   }
 
   T visitCascade(Cascade node) {
     // Ignore the result of the cascade send and return the type of the cascade
     // receiver.
     visit(node.expression);
     return cascadeReceiverStack.removeLast();
   }
 }