Add hint for use of ?. in conditions (issue 24649)

pkg/analyzer/lib/src/generated/error.dart

Index: pkg/analyzer/lib/src/generated/error.dart
diff --git a/pkg/analyzer/lib/src/generated/error.dart b/pkg/analyzer/lib/src/generated/error.dart
index af45572c7bba909c839ffd665d84963d9bba4654..66a58bcee827592e16615d3487dc8acdd0b690d1 100644
--- a/pkg/analyzer/lib/src/generated/error.dart
+++ b/pkg/analyzer/lib/src/generated/error.dart
@@ -3023,6 +3023,15 @@ class HintCode extends ErrorCode {
       "Either add a return statement or change the return type to 'void'");
 
   /**
+   * Generate a hint for conditions in control flow statements that uses the
+   * null-aware '?.' operator.
+   */
+  static const HintCode NULL_AWARE_IN_CONDITION = const HintCode(
+      'NULL_AWARE_IN_CONDITION',
+      "The value of the '?.' operator can be 'null', which is not appropriate in a condition",
+      "Replace the '?.' with a '.', testing the left-hand side for null if necessary");
+
+  /**
    * A getter with the override annotation does not override an existing getter.
    */
   static const HintCode OVERRIDE_ON_NON_OVERRIDING_GETTER = const HintCode(