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

pkg/analyzer/test/generated/compile_time_error_code_test.dart

 // Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library analyzer.test.generated.compile_time_error_code_test;
 
 import 'dart:async';
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
@@ skipping 5245 matching lines @@
 f() {
   p?.g();
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(
         source, [CompileTimeErrorCode.PREFIX_IDENTIFIER_NOT_FOLLOWED_BY_DOT]);
     verify([source]);
   }
 
+  test_privateCollisionInMixinApplication_mixinAndMixin() async {
+    addNamedSource(
+        '/lib1.dart',
+        '''
+class A {
+  int _x;
+}
+
+class B {
+  int _x;
+}
+''');
+    Source source = addSource('''
+import 'lib1.dart';
+class C extends Object with A, B {}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(
+        source, [CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION]);
+    verify([source]);
+  }
+
+  test_privateCollisionInMixinApplication_mixinAndMixin_indirect() async {
+    addNamedSource(
+        '/lib1.dart',
+        '''
+class A {
+  int _x;
+}
+
+class B {
+  int _x;
+}
+''');
+    Source source = addSource('''
+import 'lib1.dart';
+class C extends Object with A {}
+class D extends C with B {}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(
+        source, [CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION]);
+    verify([source]);
+  }
+
+  test_privateCollisionInMixinApplication_superclassAndMixin() async {
+    addNamedSource(
+        '/lib1.dart',
+        '''
+class A {
+  int _x;
+}
+
+class B {
+  int _x;
+}
+''');
+    Source source = addSource('''
+import 'lib1.dart';
+class C extends A with B {}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(
+        source, [CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION]);
+    verify([source]);
+  }
+
+  test_privateCollisionInMixinApplication_superclassAndMixin_same() async {
+    addNamedSource(
+        '/lib1.dart',
+        '''
+class A {
+  int _x;
+}
+
+class B {
+  int _x;
+}
+''');
+    Source source = addSource('''
+import 'lib1.dart';
+class C extends A with A {}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(
+        source, [CompileTimeErrorCode.PRIVATE_COLLISION_IN_MIXIN_APPLICATION]);
+    verify([source]);
+  }
+
   test_privateOptionalParameter() async {
     Source source = addSource("f({var _p}) {}");
     await computeAnalysisResult(source);
     assertErrors(source, [CompileTimeErrorCode.PRIVATE_OPTIONAL_PARAMETER]);
     verify([source]);
   }
 
   test_privateOptionalParameter_fieldFormal() async {
     Source source = addSource(r'''
 class A {
@@ skipping 1213 matching lines @@
         source, [CompileTimeErrorCode.WRONG_NUMBER_OF_PARAMETERS_FOR_OPERATOR]);
     verify([source]);
     reset();
   }
 
   Future<Null> _check_wrongNumberOfParametersForOperator1(String name) async {
     await _check_wrongNumberOfParametersForOperator(name, "");
     await _check_wrongNumberOfParametersForOperator(name, "a, b");
   }
 }