The rest of the runtime probably was already human-written in Go

find -name '*.X' | xargs wc -l in src/runtime says

Go 1.4:

    21701 .c  33348 .go 19160 .s

Go 1.7b2:

    3340 .c 77597 .go 33827 .s

The assembly growth seems to be attributable to the increased number of supported platforms