Tools:

cmd/dist: add list subcommand to list all supported platforms (CL 19837)
cmd/go: GO15VENDOREXPERIMENT gone, assumed on (CL 19615)
cmd/link: "-X name value" form gone (CL 19614)
cmd/compile: smaller binaries (many CLs)
cmd/go, go/build: add support for Fortran (CL 19670, CL 4114)
cmd/dist: introduce list subcommand to list all supported platforms (CL 19837)

Ports:

We now require OpenBSD 5.6+ (CL 18219, crypto/rand using getentropy)
plan9/arm support? Start at least.

API additions and behavior changes:

runtime: add CallerFrames and Frames (CL 19869)
testing/quick: now generates nil values (CL 16470)
net/http/httptest: ResponseRecorder supports trailer (CL 20047) (compat impact: issue 14928)
net/url: support query string without values (CL 19931)
net/textproto: permit all valid token chars in CanonicalMIMEHeaderKey input (CL 18725)
go/doc: add Unordered boolean to Example struct (CL 19280)