Revert "Add error for mixins defining conflicting private names (issue 28809)"

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

Index: pkg/analyzer/lib/src/error/codes.dart
diff --git a/pkg/analyzer/lib/src/error/codes.dart b/pkg/analyzer/lib/src/error/codes.dart
index 7dd7f445e7641b05ce63a12d722a3dbf77a66848..8171fa6ab13f4c118a0c4237092fad9db00b2bec 100644
--- a/pkg/analyzer/lib/src/error/codes.dart
+++ b/pkg/analyzer/lib/src/error/codes.dart
@@ -2046,17 +2046,6 @@ class CompileTimeErrorCode extends ErrorCode {
           "renaming the prefix.");
 
   /**
-   * It is an error for a mixin to add a private name that conflicts with a
-   * private name added by a superclass or another mixin.
-   */
-  static const CompileTimeErrorCode PRIVATE_COLLISION_IN_MIXIN_APPLICATION =
-      const CompileTimeErrorCode(
-          'PRIVATE_COLLISION_IN_MIXIN_APPLICATION',
-          "The private name {0}, defined by {1}, conflicts with the same name "
-          "defined by {2}.",
-          "Try removing {1} from the 'with' clause.");
-
-  /**
    * 6.2.2 Optional Formals: It is a compile-time error if the name of a named
    * optional parameter begins with an '_' character.
    */