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

pkg/compiler/lib/src/warnings.dart

Index: pkg/compiler/lib/src/warnings.dart
diff --git a/pkg/compiler/lib/src/warnings.dart b/pkg/compiler/lib/src/warnings.dart
index f14426292f4082c1e29bfdf0c98fb825e2b2afd0..7fd779a047102f38f95938963daf6e8a6f3fcec3 100644
--- a/pkg/compiler/lib/src/warnings.dart
+++ b/pkg/compiler/lib/src/warnings.dart
@@ -1202,7 +1202,6 @@ main() => new C();"""]);
       "Cannot assign a value to a type. Note that types are never null, "
       "so this ??= assignment has no effect.",
       howToFix: "Try removing the '??=' assignment.",
-      options: const ['--enable-null-aware-operators'],
       examples: const [
           "class A {} main() { print(A ??= 3);}",
       ]);
@@ -2022,10 +2021,6 @@ main() => r\"\"\"
       // This is a fall-back message that shouldn't happen.
       "Incomplete token.");
 
-  static const MessageKind NULL_AWARE_OPERATORS_DISABLED = const MessageKind(
-      "Null-aware operators like '#{operator}' are currently experimental. "
-      "You can enable them using the --enable-null-aware-operators flag.");
-
   static const MessageKind EXPONENT_MISSING = const MessageKind(
       "Numbers in exponential notation should always contain an exponent"
       " (an integer number with an optional sign).",