[analyzer] improve error message of

`PRIVATE_COLLISION_IN_MIXIN_APPLICATION` for setters

Bug: #49355
Change-Id: Iacd40b5c996040f279c2630a4962eed26e28546a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279719
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Brian Wilkerson <brianwilkerson@google.com>

Author: asashour

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

@@ -3606,10 +3606,12 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
     bool isConflictingName(
         String name, LibraryElement library, NamedType namedType) {
       if (Identifier.isPrivateName(name)) {
-        Map<String, String> names =
-            mixedInNames.putIfAbsent(library, () => <String, String>{});
+        Map<String, String> names = mixedInNames.putIfAbsent(library, () => {});
         var conflictingName = names[name];
         if (conflictingName != null) {
+          if (name.endsWith('=')) {
+            name = name.substring(0, name.length - 1);
+          }
           errorReporter.reportErrorForNode(
               CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION,
               namedType,
@@ -3623,6 +3625,9 @@ class ErrorVerifier extends RecursiveAstVisitor<void>
           concrete: true,
         );
         if (inheritedMember != null) {
+          if (name.endsWith('=')) {
+            name = name.substring(0, name.length - 1);
+          }
           // Inherited members are always contained inside named elements, so we
           // can safely assume `inheritedMember.enclosingElement3.name` is
           // non-`null`.

pkg/analyzer/test/src/diagnostics/private_collision_in_mixin_application_test.dart

@@ -369,7 +369,8 @@ import 'a.dart';
 
 class C extends A with B {}
 ''', [
-      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 41, 1),
+      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 41, 1,
+          messageContains: ["'_foo'"]),
     ]);
   }
 
@@ -389,7 +390,8 @@ import 'a.dart';
 
 class C extends A with B {}
 ''', [
-      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 41, 1),
+      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 41, 1,
+          messageContains: ["'_foo'"]),
     ]);
   }
 
@@ -614,7 +616,8 @@ enum E with A, B {
   v
 }
 ''', [
-      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 33, 1),
+      error(CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION, 33, 1,
+          messageContains: ["'_foo'"]),
     ]);
   }
 }