It is a tough question but I will try.
The original statement of First Law was given by Clausius and referred to cyclic processes only. He said that, and I paraphrase, the total amount of heat given to/ rejected by the system is equal to the work done by/on the system in a cyclic process. This means that the algebraic sum of heat input and work done by the system is not a path function, but a state function, and is thus a property of the system. This property is called the internal energy of the system. Thus, in a way, the definition of internal energy may be taken as a corollary of the First Law, just as the definition of entropy is taken as the corollary of the Second Law.
Having said that, I will contradict myself because this pertains to the original statement of the First Law. The modern, revised versions of the First Law have the concept of internal energy explicitly included in them, thus emancipating it from the stigma of being a corollary; just as the modern statement of Second law has the concept of entropy explicitly mentioned in it.
Tricky question, though.
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