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

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

Index: pkg/compiler/lib/src/scanner/scanner_task.dart
diff --git a/pkg/compiler/lib/src/scanner/scanner_task.dart b/pkg/compiler/lib/src/scanner/scanner_task.dart
index e2390262232974a3eb20d392da0de6d3f9aacf14..636ee34a0890615d20fd6ba2a8a696f45383557d 100644
--- a/pkg/compiler/lib/src/scanner/scanner_task.dart
+++ b/pkg/compiler/lib/src/scanner/scanner_task.dart
@@ -29,8 +29,7 @@ class ScannerTask extends CompilerTask {
   void scanElements(CompilationUnitElement compilationUnit) {
     Script script = compilationUnit.script;
     Token tokens = new Scanner(script.file,
-        includeComments: compiler.preserveComments,
-        enableNullAwareOperators: compiler.enableNullAwareOperators).tokenize();
+        includeComments: compiler.preserveComments).tokenize();
     if (compiler.preserveComments) {
       tokens = compiler.processAndStripComments(tokens);
     }