add an option to turn off errors from strong mode is-checks

pkg/dev_compiler/test/browser/runtime_tests.js

 // 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.
 
 define(['dart_sdk'], function(dart_sdk) {
   const assert = chai.assert;
   const async = dart_sdk.async;
   const core = dart_sdk.core;
   const collection = dart_sdk.collection;
   const dart = dart_sdk.dart;
@@ skipping 125 matching lines @@
       let l = dart.list([1, 2, 3], core.int);
       let s = l[dartx.join]();
       assert.equal(s, '123');
     });
   });
 
 
   suite('instanceOf', () => {
     "use strict";
 
+    setup(() => {
+      dart_sdk.dart.failForWeakModeIsChecks(true);
+    });
+
+    teardown(() => {
+      dart_sdk.dart.failForWeakModeIsChecks(false);
+    });
+
     let expect = assert.equal;
     let isGroundType = dart.isGroundType;
     let generic = dart.generic;
     let intIsNonNullable = false;
     let cast = dart.as;
     let instanceOf = dart.is;
     let strongInstanceOf = dart.strongInstanceOf;
     let getReifiedType = dart.getReifiedType;
     let functionType = dart.functionType;
     let typedef = dart.typedef;
@@ skipping 1114 matching lines @@
       list[0] = 42;
       assert.throws(() => list.add(42));
     });
 
     test('toString on ES Symbol', () => {
       let sym = Symbol('_foobar');
       assert.equal(dart.toString(sym), 'Symbol(_foobar)');
     });
   });
 });