The change introduces 2 generic mutex implementations
(futex- and semaphore-based). Each OS chooses a suitable mutex
implementation and implements few callbacks (e.g. futex wait/wake).
The CL reduces code duplication, extends some optimizations available
only on Linux/Windows to other OSes and provides ground
for futher optimizations. Chan finalizers are finally eliminated.

(Linux/amd64, 8 HT cores)
benchmark                      old      new
BenchmarkChanContended         83.6     77.8 ns/op
BenchmarkChanContended-2       341      328 ns/op
BenchmarkChanContended-4       382      383 ns/op
BenchmarkChanContended-8       390      374 ns/op
BenchmarkChanContended-16      313      291 ns/op

(Darwin/amd64, 2 cores)
benchmark                      old      new
BenchmarkChanContended         159      172 ns/op
BenchmarkChanContended-2       6735     263 ns/op
BenchmarkChanContended-4       10384    255 ns/op
BenchmarkChanCreation          1174     407 ns/op
BenchmarkChanCreation-2        4007     254 ns/op
BenchmarkChanCreation-4        4029     246 ns/op

R=rsc, jsing, hectorchu
CC=golang-dev
https://golang.org/cl/5140043

R=dsymonds
CC=golang-dev
https://golang.org/cl/5303092

R=rsc
CC=golang-dev
https://golang.org/cl/5318056

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/5307080

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5323070

A profound change to christen the new tag.

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/5327062

I promised it wouldn't last very long.
People who really need this can sync to 6a5647d82728.

««« original CL description
gc: add GOEXPERIMENT=os.Error

This won't last long, I promise.

R=ken2
CC=golang-dev
https://golang.org/cl/5330066
»»»

R=ken2
CC=golang-dev
https://golang.org/cl/5333053

This won't last long, I promise.

R=ken2
CC=golang-dev
https://golang.org/cl/5330066

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5311083

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5308077

R=adg
CC=golang-dev
https://golang.org/cl/5328062

The Plan 9 build stops in runtime,
but might as well fix these anyway.

R=adg
CC=golang-dev
https://golang.org/cl/5336045

R=adg
CC=golang-dev
https://golang.org/cl/5303091

R=adg
CC=golang-dev
https://golang.org/cl/5333052

R=golang-dev, iant, r, r
CC=golang-dev
https://golang.org/cl/5307066

R=golang-dev, bsiegert, iant
CC=golang-dev
https://golang.org/cl/5294074

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5322051

R=adg, rsc
CC=golang-dev
https://golang.org/cl/5316068

The existing code uses *os.Waitmsg as an os.Error,
but *os.Waitmsg is really just a stringer.

Introduce an explicit error type for the real error.

Not to be submitted until just before error goes in;
the gofix for error updates type assertions
        err.(*os.Waitmsg)
to
        err.(*exec.ExitError)

The seemingly redundant String method will become
an Error method when error goes in, and will no longer
be redundant.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5331044

(The definition of ErrorList is in another file, so gofix
has no hope of getting this right.)

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5330043

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5319057

R=r
CC=golang-dev
https://golang.org/cl/5298073

R=r, r
CC=golang-dev
https://golang.org/cl/5311068

R=golang-dev, iant, r
CC=golang-dev
https://golang.org/cl/5306075

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5327051

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5331046

The only function is errors.New, at least for now.

R=r, gustavo, adg, r
CC=golang-dev
https://golang.org/cl/5321061

R=ken
CC=golang-dev
https://golang.org/cl/5331043

R=adg
CC=golang-dev
https://golang.org/cl/5316070

To make the error fix more useful, expand typecheck to gather
more information about struct fields, typecheck range statements,
typecheck indirect and index of named types, and collect information
about assignment conversions.

Also, change addImport to rename top-level uses of a to-be-imported
identifier to avoid conflicts.  This duplicated some of the code in
the url fix, so that fix is now shorter.

R=iant, r, r
CC=golang-dev
https://golang.org/cl/5305066

R=golang-dev, dsymonds, r, r
CC=golang-dev
https://golang.org/cl/5308072

R=golang-dev, rsc, dsymonds
CC=golang-dev
https://golang.org/cl/5305085

R=rsc, r
CC=golang-dev
https://golang.org/cl/5336044

A <a> tag generates implied end tags for any open <a> elements.
But it shouldn't do that when it is inside a table cell the the open <a>
is outside the table.
So stop the search for an open <a> when we reach a scope marker node.

Pass tests1.dat, test 78:
<a href="blah">aba<table><tr><td><a href="foo">br</td></tr>x</table>aoe

| <html>
|   <head>
|   <body>
|     <a>
|       href="blah"
|       "abax"
|       <table>
|         <tbody>
|           <tr>
|             <td>
|               <a>
|                 href="foo"
|                 "br"
|       "aoe"

Also pass test 79:
<table><a href="blah">aba<tr><td><a href="foo">br</td></tr>x</table>aoe

R=nigeltao
CC=golang-dev
https://golang.org/cl/5320063

R=andybalholm
CC=golang-dev, mikesamuel
https://golang.org/cl/5331056

This fixes the issue without an extra copy in the average case.

R=golang-dev, ality, bradfitz
CC=golang-dev
https://golang.org/cl/5272049

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5334044

R=lvd
CC=golang-dev
https://golang.org/cl/5333049

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5331055

R=r, golang-dev, adg
CC=golang-dev
https://golang.org/cl/5308071