fixes #25610, infer JS builtin in strong mode like we used to

pkg/analyzer/test/src/task/strong/strong_test_helper.dart

 // Copyright (c) 2015, 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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 library analyzer.test.src.task.strong.strong_test_helper;
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
@@ skipping 100 matching lines @@
         class HtmlElement {}
         ''',
   'dart:math': '''
         library dart.math;
         class Random {
           bool nextBool() {}
         }
         num/*=T*/ min/*<T extends num>*/(num/*=T*/ a, num/*=T*/ b) => null;
         num/*=T*/ max/*<T extends num>*/(num/*=T*/ a, num/*=T*/ b) => null;
         ''',
+
+  'dart:_foreign_helper': '''
+  library dart._foreign_helper;
+
+  JS(String typeDescription, String codeTemplate,
+    [arg0, arg1, arg2, arg3, arg4, arg5, arg6, arg7, arg8, arg9, arg10, arg11])
+  {}
+  '''
 };
 
 /// Returns an ANSII color escape sequence corresponding to [levelName]. Colors
 /// are defined for: severe, error, warning, or info. Returns null if the level
 /// name is not recognized.
 String colorOf(String levelName) {
   levelName = levelName.toLowerCase();
   if (levelName == 'shout' || levelName == 'severe' || levelName == 'error') {
     return _RED_COLOR;
   }
@@ skipping 385 matching lines @@
   UriKind get uriKind => UriKind.DART_URI;
 
   bool exists() => true;
 
   Source resolveRelative(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 
   Uri resolveRelativeUri(Uri relativeUri) =>
       throw new UnsupportedError('not expecting relative urls in dart: mocks');
 }