Testing support for FF and Safari

pkg/dev_compiler/lib/js/common/dart_sdk.js

Index: pkg/dev_compiler/lib/js/common/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/common/dart_sdk.js b/pkg/dev_compiler/lib/js/common/dart_sdk.js
index f8258be836f3ef50351c2721accf18524f46a6e4..1b39b697b1cb5d79f66dd7fcabf796d40cf6624e 100644
--- a/pkg/dev_compiler/lib/js/common/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/common/dart_sdk.js
@@ -1021,6 +1021,7 @@
     if (!jsType) return;
     let extProto = dartExtType.prototype;
     let jsProto = jsType.prototype;
+    if (!jsProto) return;
     jsProto[dart._extensionType] = dartExtType;
     dart._installProperties(jsProto, extProto);
     let originalSigFn = dart.getOwnPropertyDescriptor(dartExtType, dart._methodSig).get;
@@ -1761,7 +1762,7 @@
   };
   dart.nullSafe = function(obj, ...callbacks) {
     if (obj == null) return obj;
-    for (const callback of callbacks) {
+    for (let callback of callbacks) {
       obj = callback(obj);
       if (obj == null) break;
     }