Implement support for overridden elements that are declared in mixins.

R=brianwilkerson@google.com

Change-Id: I70ca606b23b928481f24d1322cfbd01a9b3d3e5f
Reviewed-on: https://dart-review.googlesource.com/54071
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>

Author: scheglov

pkg/analysis_server/lib/src/computer/computer_overrides.dart

@@ -160,7 +160,7 @@ class _OverriddenElementsFinder {
    */
   OverriddenElements find() {
     _visited.clear();
-    _addSuperOverrides(_class.supertype);
+    _addSuperOverrides(_class.type, withThisType: false);
     _visited.clear();
     _addInterfaceOverrides(_class.type, false);
     _superElements.forEach(_interfaceElements.remove);
@@ -189,20 +189,23 @@ class _OverriddenElementsFinder {
     _addInterfaceOverrides(type.superclass, checkType);
   }
 
-  void _addSuperOverrides(InterfaceType type) {
+  void _addSuperOverrides(InterfaceType type, {bool withThisType: true}) {
     if (type == null) {
       return;
     }
     if (!_visited.add(type)) {
       return;
     }
-    // this type
-    Element element = _lookupMember(type.element);
-    if (element != null && !_superElements.contains(element)) {
-      _superElements.add(element);
+
+    if (withThisType) {
+      Element element = _lookupMember(type.element);
+      if (element != null && !_superElements.contains(element)) {
+        _superElements.add(element);
+      }
     }
-    // super
+
     _addSuperOverrides(type.superclass);
+    type.mixins.forEach(_addSuperOverrides);
   }
 
   Element _lookupMember(ClassElement classElement) {

pkg/analysis_server/test/analysis/notification_overrides_test.dart

@@ -405,6 +405,55 @@ class C extends A implements A {
     expect(override.interfaceMembers, isNull);
   }
 
+  test_mixin_method_direct() async {
+    addTestFile('''
+class A {
+  m() {} // in A
+}
+class B extends Object with A {
+  m() {} // in B
+}
+''');
+    await prepareOverrides();
+    assertHasOverride('m() {} // in B');
+    assertHasSuperElement('m() {} // in A');
+    assertNoInterfaceMembers();
+  }
+
+  test_mixin_method_indirect() async {
+    addTestFile('''
+class A {
+  m() {} // in A
+}
+class B extends A {
+}
+class C extends Object with B {
+  m() {} // in C
+}
+''');
+    await prepareOverrides();
+    assertHasOverride('m() {} // in C');
+    assertHasSuperElement('m() {} // in A');
+    assertNoInterfaceMembers();
+  }
+
+  test_mixin_method_indirect2() async {
+    addTestFile('''
+class A {
+  m() {} // in A
+}
+class B extends Object with A {
+}
+class C extends B {
+  m() {} // in C
+}
+''');
+    await prepareOverrides();
+    assertHasOverride('m() {} // in C');
+    assertHasSuperElement('m() {} // in A');
+    assertNoInterfaceMembers();
+  }
+
   test_staticMembers() async {
     addTestFile('''
 class A {