Look into superclassConstraints while searching for a member in inter…

…faces.

R=brianwilkerson@google.com, paulberry@google.com

Bug: #34564
Change-Id: I11642d89710a89fb347106ff374ff0ea4227310c
Reviewed-on: https://dart-review.googlesource.com/76284
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Paul Berry <paulberry@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

pkg/analyzer/lib/src/dart/element/type.dart

@@ -1508,9 +1508,11 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
     //
     List<DartType> jArgs = j.typeArguments;
     List<DartType> jVars = jElement.type.typeArguments;
-    supertype = supertype.substitute2(jArgs, jVars);
-    if (supertype == i) {
-      return true;
+    if (supertype != null) {
+      supertype = supertype.substitute2(jArgs, jVars);
+      if (supertype == i) {
+        return true;
+      }
     }
     //
     // I is listed in the on clause of J.
@@ -2394,6 +2396,13 @@ class InterfaceTypeImpl extends TypeImpl implements InterfaceType {
         return member;
       }
     }
+    for (InterfaceType constraint in targetType.superclassConstraints) {
+      ExecutableElement member = _lookUpMemberInInterfaces(
+          constraint, true, library, visitedInterfaces, getMember);
+      if (member != null) {
+        return member;
+      }
+    }
     for (InterfaceType mixinType in targetType.mixins.reversed) {
       ExecutableElement member = _lookUpMemberInInterfaces(
           mixinType, true, library, visitedInterfaces, getMember);

pkg/analyzer/test/src/dart/resolution/mixin_test.dart

@@ -1579,6 +1579,30 @@ mixin M implements A, B {}
     ]);
   }
 
+  test_isMoreSpecificThan() async {
+    addTestFile(r'''
+mixin M {}
+''');
+    await resolveTestFile();
+    assertNoTestErrors();
+
+    var element = findElement.mixin('M');
+    var type = element.type;
+    expect(type.isMoreSpecificThan(intType), isFalse);
+  }
+
+  test_lookUpMemberInInterfaces_Object() async {
+    addTestFile(r'''
+class Foo {}
+
+mixin UnhappyMixin on Foo {
+  String toString() => '$runtimeType';
+}
+''');
+    await resolveTestFile();
+    assertNoTestErrors();
+  }
+
   test_metadata() async {
     addTestFile(r'''
 const a = 0;