async_wrap: update utility methods for flexibility

This makes some of the internal `AsyncWrap::MakeCallback()`
utility wrappers throw rather than crash the process when
the requested method is not present on the `this` object.

Doing so makes it easier for future code to expose C++
objects directly to userland, where JS code can overwrite
or delete such methods.

PR-URL: ayojs#82

Author: addaleax

src/async-wrap-inl.h

@@ -50,8 +50,13 @@ inline v8::MaybeLocal<v8::Value> AsyncWrap::MakeCallback(
     const v8::Local<v8::String> symbol,
     int argc,
     v8::Local<v8::Value>* argv) {
-  v8::Local<v8::Value> cb_v = object()->Get(symbol);
-  CHECK(cb_v->IsFunction());
+  v8::Local<v8::Value> cb_v;
+  if (!object()->Get(object()->CreationContext(), symbol).ToLocal(&cb_v))
+    return v8::MaybeLocal<v8::Value>();
+  if (!cb_v->IsFunction()) {
+    env()->ThrowError("callback must be a function");
+    return v8::MaybeLocal<v8::Value>();
+  }
   return MakeCallback(cb_v.As<v8::Function>(), argc, argv);
 }
 
@@ -60,8 +65,13 @@ inline v8::MaybeLocal<v8::Value> AsyncWrap::MakeCallback(
     uint32_t index,
     int argc,
     v8::Local<v8::Value>* argv) {
-  v8::Local<v8::Value> cb_v = object()->Get(index);
-  CHECK(cb_v->IsFunction());
+  v8::Local<v8::Value> cb_v;
+  if (!object()->Get(object()->CreationContext(), index).ToLocal(&cb_v))
+    return v8::MaybeLocal<v8::Value>();
+  if (!cb_v->IsFunction()) {
+    env()->ThrowError("callback must be a function");
+    return v8::MaybeLocal<v8::Value>();
+  }
   return MakeCallback(cb_v.As<v8::Function>(), argc, argv);
 }