Tools:

cmd/cgo: fix C.complexfloat and C.complexdouble and treat as separate types from complex64 and complex128 (https://golang.org/cl/17208)
cmd/cgo: new pointer passing rules defined and enforced (https://golang.org/issue/12416, https://golang.org/issue/17064)
cmd/compile: parser now hand-written
cmd/compile: add -msan option (https://golang.org/cl/16160)
cmd/dist: use clang on FreeBSD (https://golang.org/cl/16635)
cmd/doc: search packages in breadth-first order (https://golang.org/cl/17691)
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/go: -buildmode=pie now default on android/arm (https://golang.org/cl/16055)
cmd/link: add -libgcc option (https://golang.org/cl/16993)
cmd/link: add -msan option (https://golang.org/cl/16161)
cmd/vet: diagnose using Printf on a func value
cmd/vet: -all -shadow means all the default checks and the shadow check (https://golang.org/cl/16325)

Ports:

Add new experimental ports for linux/mips64 and linux/mips64le: no cgo, external linking or disasm yet (https://golang.org/cl/14460 and others)
NaCl is no longer restricted to pepper_41 (https://golang.org/cl/13958/)

cmd/go: new -buildmode=shared, -buildmode=pie

CX no longer available for 386 assembly? (https://golang.org/cl/16386)

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:

archive/zip: per-file compressors (https://golang.org/cl/16669)
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)
compress/bzip2: about 20% faster to decode (https://golang.org/cl/13853)
compress/flate: deprecate ReadError, WriteError (https://golang.org/cl/14834)
compress/flate: detect truncated streams (https://golang.org/cl/14833)
compress/gzip: detect truncated streams (https://golang.org/cl/14832)
compress/zlib: detect truncated streams (https://golang.org/cl/14835)
crypto/aes: dedicated asm version of AES-GCM (https://golang.org/cl/10484)
crypto/elliptic, ecrypto/ecdsa: P256 assembly (https://golang.org/cl/8968)
crypto/tls: allow tls.Listen when only GetCertifite is provided (https://golang.org/cl/13801)
crypto/tls: support for TSA_RSA_WITH_AES_128_GCM_SHA256 and TLS_RSA_WITH_AES_256_GCM_SHA384 (https://golang.org/cl/16924)
crypto/tls: RecordHeaderError (https://golang.org/cl/16078)
crypto/x509: permit negative serial numbers (https://golang.org/cl/17247)
crypto/x509: InsecureAlgorithmError (https://golang.org/cl/17400)
debug/elf: supports old-style compressed DWARF (https://golang.org/cl/17340)
debug/elf: transparent decompress of compressed sections (https://golang.org/cl/17341)
encoding/asn1: export tag and class constants (https://golang.org/cl/17311)
encoding/asn1: enforce use of short form lengths (https://golang.org/cl/16517)
encoding/asn1: reject invalid integers (https://golang.org/cl/17008)
encoding/json: Number marshaling check validity (https://golang.org/cl/12250)
encoding/xml: ,cdata attribute (https://golang.org/cl/16047)
encoding/xml: SyntaxError for prematurely ended XML (https://golang.org/cl/14315)
fmt: allow any integer type as an argument to the * operator (https://golang.org/cl/14491/)
fmt: scan %X for strings (https://golang.org/cl/15689)
image: add NYCbCrA types (https://golang.org/cl/15671)
io: MultiWriter now supports WriteString (https://golang.org/cl/12485)
math/big: new API Int.Text, Int.Append (https://golang.org/cl/14994)
math/big: Float implements TextMarshaler, TextUnmarshaler (https://golang.org/cl/15050)
math/big: -1 precision now works for float.Append
math/rand: add Read (https://golang.org/cl/14522)
net: ParseMAC now accepts 20-byte IPoIB link-layer addresses
net: DNS server error now reported as Temporary (https://golang.org/cl/14169)
net: rooted DNS names on Windows (https://golang.org/cl/13887), Plan 9 (https://golang.org/cl/15581)
net: https://golang.org/cl/17216 is "localhost." (possible bug; see https://golang.org/issue/13564)
net/http: new error codes from RFC 6585 (https://golang.org/cl/15732)
net/http: Client supports Expect: 100-continue (https://golang.org/cl/10091)
net/http: HTTP/2.0 support (many CLs)
net/http: FileServer now sorts directory entries (https://golang.org/cl/14161)
net/http: ResponseRecorder detects Content-Type (https://golang.org/cl/16096)
net/http: req.Method=="" now follows redirects like req.Method=="GET" (https://golang.org/cl/17318)
net/http: make NewRequest with empty method set Method==GET (https://golang.org/cl/17699)
net/url: make *url.Error implement net.Error (https://golang.org/cl/15672)
net/url: processing of host name stricter, more spec compliant (https://golang.org/cl/17387)
os: IsPermission, IsExist, and IsNotExist now handle *os.SyscallError (https://golang.org/cl/15730)
os/exec: ExitError includes stderr (https://golang.org/cl/11415)
path/filepath: EvalSymlinks changed (https://golang.org/cl/16192); possible bug (https://golang.org/issue/13582)
path/filepath: Join("c:", "a") now returns `c:a` instead of `c:\a` on windows (https://golang.org/cl/17470)
regexp: Copy method (https://golang.org/cl/16110)
runtime: GODEBUG=cgocheck=2 (https://golang.org/cl/16899)
runtime: only one goroutine in traceback (https://golang.org/cl/16512) maybe
runtime: best-effort detection of concurrent misuse of maps (https://golang.org/cl/17501)
sort: faster but different order (https://golang.org/cl/15688, https://golang.org/cl/17389)
strconv: QuoteToGraphic (https://golang.org/cl/14184/)
testing: pause test timer during t.Parallel (https://golang.org/cl/16989)
testing/quick: terminates for recursive types (https://golang.org/cl/13830)
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: reject invalid day of month in Parse (https://golang.org/cl/17710)