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

pkg/compiler/lib/src/dart2js.dart

Index: pkg/compiler/lib/src/dart2js.dart
diff --git a/pkg/compiler/lib/src/dart2js.dart b/pkg/compiler/lib/src/dart2js.dart
index 2cbaf313fa8596f24c025d1bd7cda2c89e7a8529..51cdae407221e695a022d05cc7af05e96247600a 100644
--- a/pkg/compiler/lib/src/dart2js.dart
+++ b/pkg/compiler/lib/src/dart2js.dart
@@ -359,7 +359,12 @@ Future<api.CompilationResult> compile(List<String> argv) {
           "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. "