Testing support for FF and Safari

pkg/dev_compiler/lib/js/amd/dart_sdk.js

Index: pkg/dev_compiler/lib/js/amd/dart_sdk.js
diff --git a/pkg/dev_compiler/lib/js/amd/dart_sdk.js b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
index 7d8c1b8aef219b71a17aca98d22e995ed038642d..8ec4be2dda22ca975efb73a443b011f536b9a7f4 100644
--- a/pkg/dev_compiler/lib/js/amd/dart_sdk.js
+++ b/pkg/dev_compiler/lib/js/amd/dart_sdk.js
@@ -1021,6 +1021,7 @@ define([], function() {
     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 @@ define([], function() {
   };
   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;
     }