The frontend is usually the hard part IMO. In WebKit, we have 100,000 lines of code for our "frontend" (i.e. the DFG compiler) and 50,000 lines of code for our "backend" (i.e. B3). That split is really interesting.

Is that really the case? It seems to me that a Prolog-like resolution system, coupled with a constraint solver, would get most of the job done with little effort.

There are certainly many rules to keep track of, but in some cases the newer rules are strict generalisations of the older rules. For example, Haskell's original type classes are just a special-case of modern multi-parameter, flexible instances/contexts, etc. type classes. Likewise, we can implement constraint kinds, type class instance resolution and type equalities using one constraint solver.

To me this is the difference between knowing that your program will probably work after it compiles, vs not knowing until after its deployed.

What I think people really enjoy in dynamic languages like JavaScript is the instant feedback, just reloading the browser. This can be accomplished with decent IDEs for most statically typed languages.