Remove flag for enabling null-aware operators. They are now on by default.

pkg/compiler/lib/src/dart2js.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 dart2js.cmdline;
 
 import 'dart:async'
     show Future, EventSink;
 import 'dart:convert' show UTF8, LineSplitter;
 import 'dart:io'
@@ skipping 341 matching lines @@
                       (_) => hasDisallowUnsafeEval = true),
     new OptionHandler('--show-package-warnings', passThrough),
     new OptionHandler('--csp', passThrough),
     new OptionHandler('--enable-experimental-mirrors', passThrough),
     new OptionHandler('--enable-async', (_) {
       diagnosticHandler.info(
           "Option '--enable-async' is no longer needed. "
           "Async-await is supported by default.",
           api.Diagnostic.HINT);
     }),
-    new OptionHandler('--enable-null-aware-operators', passThrough),
+    new OptionHandler('--enable-null-aware-operators',  (_) {
+      diagnosticHandler.info(
+          "Option '--enable-null-aware-operators' is no longer needed. "
+          "Null aware operators are supported by default.",
+          api.Diagnostic.HINT);
+    }),
     new OptionHandler('--enable-enum', (_) {
       diagnosticHandler.info(
           "Option '--enable-enum' is no longer needed. "
           "Enums are supported by default.",
           api.Diagnostic.HINT);
     }),
     new OptionHandler('--allow-native-extensions', setAllowNativeExtensions),
     new OptionHandler('--generate-code-with-compile-time-errors', passThrough),
     new OptionHandler('--test-mode', passThrough),
 
@@ skipping 387 matching lines @@
       } else if (exitCode == 253) {
         print(">>> TEST CRASH");
       } else {
         print(">>> TEST FAIL");
       }
       stderr.writeln(">>> EOF STDERR");
       subscription.resume();
     });
   });
 }