Tools:

cmd/go: vendoring enabled by default (https://golang.org/cl/13967/)
cmd/go: flags for tests must precede package name if present; also makes it easier to pass flags to test binaries (https://golang.org/cl/14826)
cmd/go: add -msan option (https://golang.org/cl/16169)
cmd/compile: add -msan option (https://golang.org/cl/16160)
cmd/link: add -msan option (https://golang.org/cl/16161)

Ports:

NaCl is no longer restricted to pepper_41 (https://golang.org/cl/13958/)

Reflect change:
cmd/compile/internal/gc: make embedded unexported structs RO (https://golang.org/cl/14085)
encoding/json: check for exported fields in embedded structs (https://golang.org/cl/14011)
encoding/xml: check for exported fields in embedded structs (https://golang.org/cl/14012)
reflect: adjust access to unexported embedded structs (https://golang.org/cl/14010)

API additions and behavior changes:

bufio: add Scanner.Buffer (https://golang.org/cl/14599/)
bufio: add ErrFinalToken as a sentinel value for Scan's split functions (https://golang.org/cl/14924)
crypto/aes: dedicated asm version of AES-GCM (https://golang.org/cl/10484)
fmt: allow any integer type as an argument to the * operator (https://golang.org/cl/14491/)
image: add NYCbCrA types (https://golang.org/cl/15671)
math/rand: add Read (https://golang.org/cl/14522)
net/http: HTTP/2.0 support (many CLs)
net/url: make *url.Error implement net.Error (https://golang.org/cl/15672)
runtime: only one goroutine in traceback (https://golang.org/cl/16512) maybe
strconv: QuoteToGraphic (https://golang.org/cl/14184/)
text/template: ExecError (https://golang.org/cl/13957/)
text/template: trimming spaces (https://golang.org/cl/14391/)
text/template: Funcs check names (https://golang.org/cl/14562/)
text/template: IsTrue (https://golang.org/cl/14562/)
text/template: blocks and permit redefinition (https://golang.org/cl/14005)
time: allow one and two-digit days of the month during Parse (https://golang.org/cl/14123/)