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

pkg/compiler/lib/src/apiimpl.dart

Index: pkg/compiler/lib/src/apiimpl.dart
diff --git a/pkg/compiler/lib/src/apiimpl.dart b/pkg/compiler/lib/src/apiimpl.dart
index 9fd865acec57283e887d4bde311919d1f3593546..3c493edd9ee49f243d62352e2282675b24dad45c 100644
--- a/pkg/compiler/lib/src/apiimpl.dart
+++ b/pkg/compiler/lib/src/apiimpl.dart
@@ -104,9 +104,7 @@ class Compiler extends leg.Compiler {
                 hasOption(options, '--generate-code-with-compile-time-errors'),
             testMode: hasOption(options, '--test-mode'),
             allowNativeExtensions:
-                hasOption(options, '--allow-native-extensions'),
-            enableNullAwareOperators:
-                hasOption(options, '--enable-null-aware-operators')) {
+                hasOption(options, '--allow-native-extensions')) {
     tasks.addAll([
         userHandlerTask = new leg.GenericTask('Diagnostic handler', this),
         userProviderTask = new leg.GenericTask('Input provider', this),