Comment snippets from Jacob:

Requested Dart API:
"LibraryHasMember" or something similar.

Basically we need an internal and external version depending on whether the
question you are asking is:
"Does library Foo have member Bar when used externally and does it have member
Bar when you are within library Bar itself.
   return !Dart_IsError(Dart_GetField(library, name))
   83 || !Dart_IsError(Dart_GetClass(library, name))
   84 || Dart_IsFunction(Dart_LookupFunction(library, name));

Requested Dart Api:
"ClassHasMember"
would be nice if this was one line.
return !Dart_IsError(Dart_GetField(clazz, name)) || Dart_IsFunction(Dart_Loo
      kupFunction(clazz, name));