3.2 Find the dependent variable of an ode

Given a single ode, how to find the dependent variable?

Internal`ProcessEquations`FindDependentVariables[y''[x] + y[x]*y'[x] == x, x] 
 
   {y}
 

Internal`ProcessEquations`FindDependentVariables[x'[t] == 0, t] 
 
   {x}
 

Notice the second argument must be the independent variable.