spec: clarify when range x does not evaluate x

Fixes issue 4644.

R=r, adonovan
CC=golang-dev
https://codereview.appspot.com/7307083
cmd/go: reject case-insensitive file name, import collisions

To make sure that Go code will work when moved to a
system with a case-insensitive file system, like OS X or Windows,
reject any package built from files with names differing
only in case, and also any package built from imported
dependencies with names differing only in case.

Fixes issue 4773.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7314104
runtime: make return from main wait for active panic

Arguably if this happens the program is buggy anyway,
but letting the panic continue looks better than interrupting it.
Otherwise things like this are possible, and confusing:

$ go run x.go
panic: $ echo $?
0
$

Fixes issue 3934.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7322083
runtime: show frame pointer values during throw

Should help if stack overflows start happening again.

Fixes issue 3582.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7311098
runtime: allow mem profiles with GOGC=off

Fixes issue 3586.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7304098
api: refresh next.txt

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7317049
build: clang support

This works with at least one version of clang
that existed at one moment in time.
No guarantees about clangs past or future.

To try:
        CC=clang all.bash

It does not work with the Xcode clang,
because that clang fails at printing a useful answer
to:
        clang -print-libgcc-file-name
The clang that works prints a full path name for
that command, not just "libgcc.a".

Fixes issue 4713.

R=iant, minux.ma
CC=golang-dev
https://codereview.appspot.com/7323068
os: point users of ProcessState.SysUsage to getrusage(2)

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/7352044
undo CL 7310096 / 59da6744d66d

broke windows build

««« original CL description
runtime: ensure forward progress of runtime.Gosched() for locked goroutines
The removed code leads to the situation when M executes the same locked G again and again.
Fixes issue 4820.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7310096
»»»

TBR=dvyukov
CC=golang-dev
https://codereview.appspot.com/7343050
runtime: fix unused variable warning

R=rsc, dvyukov
CC=golang-dev
https://codereview.appspot.com/7312103
net: add IPConn through Conn test

Also refactors mock ICMP stuff.

R=dave, rsc
CC=golang-dev
https://codereview.appspot.com/7325043