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

pkg/compiler/lib/src/compiler.dart

Index: pkg/compiler/lib/src/compiler.dart
diff --git a/pkg/compiler/lib/src/compiler.dart b/pkg/compiler/lib/src/compiler.dart
index 8360a5dbe422d16b8ca813293b2a4bd503ae4ae4..0cc5a67947448aa112ddb1903141d2617f098970 100644
--- a/pkg/compiler/lib/src/compiler.dart
+++ b/pkg/compiler/lib/src/compiler.dart
@@ -791,10 +791,6 @@ abstract class Compiler implements DiagnosticListener {
   /// If `true` native extension syntax is supported by the frontend.
   final bool allowNativeExtensions;
 
-  /// Temporary flag to enable `?.`, `??`, and `??=` until it becomes part of
-  /// the spec.
-  final bool enableNullAwareOperators;
-
   /// Output provider from user of Compiler API.
   api.CompilerOutputProvider userOutputProvider;
 
@@ -1063,7 +1059,6 @@ abstract class Compiler implements DiagnosticListener {
             bool hasIncrementalSupport: false,
             this.enableExperimentalMirrors: false,
             this.allowNativeExtensions: false,
-            this.enableNullAwareOperators: false,
             this.generateCodeWithCompileTimeErrors: false,
             this.testMode: false,
             api.CompilerOutputProvider outputProvider,