simplify emitting dart.bind for native types

lib/runtime/dart_sdk.js

Index: lib/runtime/dart_sdk.js
diff --git a/lib/runtime/dart_sdk.js b/lib/runtime/dart_sdk.js
index 3b83779fd143ceb9e955fb6568ae3c810f63148f..92124c18e07785ba58873d2bf7ceeeaac3f5690c 100644
--- a/lib/runtime/dart_sdk.js
+++ b/lib/runtime/dart_sdk.js
@@ -114,9 +114,8 @@ dart_library.library('dart_sdk', null, /* Imports */[
   dart._methodSig = Symbol("sig");
   dart._staticSig = Symbol("sigStatic");
   dart.getMethodType = function(obj, name) {
-    if (obj === void 0) return void 0;
-    if (obj == null) return void 0;
-    return dart.getMethodTypeFromType(obj.__proto__.constructor, name);
+    let type = obj == null ? core.Object : obj.__proto__.constructor;
+    return dart.getMethodTypeFromType(type, name);
   };
   dart.getMethodTypeFromType = function(type, name) {
     let sigObj = type[dart._methodSig];