fix override checking of mixins

tests/corelib_2/string_replace_test.dart

 // Copyright (c) 2011, 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.
 
 import "package:expect/expect.dart";
 
 main() {
   // Test replaceFirst.
   Expect.equals("AtoBtoCDtoE", "AfromBtoCDtoE".replaceFirst("from", "to"));
 
@@ skipping 157 matching lines @@
 
   // Test replacement throwing.
   Expect.throws(() => "foo-bar".replaceFirstMapped("bar", (v) => throw 42),
       (e) => e == 42);
 
   // Test replacement returning non-String.
   var o = new Object();
   Expect.equals(
       "foo-$o",
       "foo-bar".replaceFirstMapped("bar", (v) {
-        return o;
+        // TODO(jmesserly): in strong mode, this function must return a string.
+        // If we want to allow any Object, we'll have to fix the API signature.
+        // See https://github.com/dart-lang/sdk/issues/30248.
+        return '$o';
       }));
 
   for (var string in ["", "x", "foo", "x\u2000z"]) {
     for (var replacement in ["", "foo", string]) {
       for (int start = 0; start <= string.length; start++) {
         var expect;
         for (int end = start; end <= string.length; end++) {
           expect =
               string.substring(0, start) + replacement + string.substring(end);
           Expect.equals(expect, string.replaceRange(start, end, replacement),
               '"$string"[$start:$end]="$replacement"');
         }
         // Reuse expect from "end == string.length" case when omitting end.
         Expect.equals(expect, string.replaceRange(start, null, replacement),
             '"$string"[$start:]="$replacement"');
       }
     }
     Expect.throws(() => string.replaceRange(0, 0, null));
     Expect.throws(() => string.replaceRange(-1, 0, "x"));
     Expect.throws(() => string.replaceRange(0, string.length + 1, "x"));
   }
 }