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

pkg/dev_compiler/test/browser/runtime_tests.js

Index: pkg/dev_compiler/test/browser/runtime_tests.js
diff --git a/pkg/dev_compiler/test/browser/runtime_tests.js b/pkg/dev_compiler/test/browser/runtime_tests.js
index 6ed5f16ca2c20929172d3a06e232e8de50fc3f3b..48c31d43b631929b43f86aaabd5f6aaa35914031 100644
--- a/pkg/dev_compiler/test/browser/runtime_tests.js
+++ b/pkg/dev_compiler/test/browser/runtime_tests.js
@@ -143,6 +143,14 @@ define(['dart_sdk'], function(dart_sdk) {
   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;