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

pkg/compiler/lib/src/scanner/listener.dart

Index: pkg/compiler/lib/src/scanner/listener.dart
diff --git a/pkg/compiler/lib/src/scanner/listener.dart b/pkg/compiler/lib/src/scanner/listener.dart
index b6c356a0a98b398420120888688ecb8869e2e68a..ef9b651dee4af92af952e4813aa4b7865dcb3173 100644
--- a/pkg/compiler/lib/src/scanner/listener.dart
+++ b/pkg/compiler/lib/src/scanner/listener.dart
@@ -746,9 +746,6 @@ class Listener {
           break;
       }
       reportError(token, kind, arguments);
-    } else if (token is UnsupportedNullAwareToken) {
-      reportError(token, MessageKind.NULL_AWARE_OPERATORS_DISABLED,
-          {'operator' : token.operator});
     } else if (token is UnmatchedToken) {
       String begin = token.begin.value;
       String end = closeBraceFor(begin);