contribute local completions for mixin members

Fixes: dart-lang/sdk#39795

Change-Id: I1a186dd63e59463706d18dd484653c0bee70ba90
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/132165
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Phil Quitslund <pquitslund@google.com>

pkg/analysis_server/test/services/completion/dart/local_reference_contributor_test.dart

@@ -4050,6 +4050,32 @@ A T;''');
     assertNotSuggested('x');
   }
 
+  test_mixinDeclaration_method_access() async {
+    addTestSource(r'''
+class A { }
+
+mixin X on A {
+  int _x() => 0;
+  int get x => ^
+}
+''');
+    await computeSuggestions();
+    assertSuggestMethod('_x', 'X', 'int');
+  }
+
+  test_mixinDeclaration_property_access() async {
+    addTestSource(r'''
+class A { }
+
+mixin X on A {
+  int _x;
+  int get x => ^
+}
+''');
+    await computeSuggestions();
+    assertSuggestField('_x', 'int');
+  }
+
   test_new_instance() async {
     addTestSource('import "dart:math"; class A {x() {new Random().^}}');
     await computeSuggestions();

pkg/analyzer_plugin/lib/src/utilities/visitors/local_declaration_visitor.dart

@@ -97,7 +97,7 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
 
   @override
   void visitClassDeclaration(ClassDeclaration node) {
-    _visitClassDeclarationMembers(node);
+    _visitClassOrMixinMembers(node);
     visitNode(node);
   }
 
@@ -146,13 +146,13 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
   }
 
   @override
-  visitConstructorDeclaration(ConstructorDeclaration node) {
+  void visitConstructorDeclaration(ConstructorDeclaration node) {
     _visitParamList(node.parameters);
     visitNode(node);
   }
 
   @override
-  visitForStatement(ForStatement node) {
+  void visitForStatement(ForStatement node) {
     var forLoopParts = node.forLoopParts;
     if (forLoopParts is ForEachPartsWithDeclaration) {
       DeclaredIdentifier loopVariable = forLoopParts.loopVariable;
@@ -199,6 +199,12 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
     visitNode(node);
   }
 
+  @override
+  void visitMixinDeclaration(MixinDeclaration node) {
+    _visitClassOrMixinMembers(node);
+    visitNode(node);
+  }
+
   @override
   void visitNode(AstNode node) {
     // Support the case of searching partial ASTs by aborting on nodes with no
@@ -227,7 +233,7 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
     visitNode(node);
   }
 
-  void _visitClassDeclarationMembers(ClassDeclaration node) {
+  void _visitClassOrMixinMembers(ClassOrMixinDeclaration node) {
     for (ClassMember member in node.members) {
       if (member is FieldDeclaration) {
         member.fields.variables.forEach((VariableDeclaration varDecl) {
@@ -263,7 +269,7 @@ abstract class LocalDeclarationVisitor extends GeneralizingAstVisitor {
     }
   }
 
-  _visitStatements(NodeList<Statement> statements) {
+  void _visitStatements(NodeList<Statement> statements) {
     for (Statement stmt in statements) {
       if (stmt.offset < offset) {
         if (stmt is VariableDeclarationStatement) {