[dev.typealias] go/build: add go1.9 build tag

It's earlier than usual but this will help us put the type alias-aware
code into x/tools without breaking clients on go1.6, go1.7,
or (eventually) go1.8.

Change-Id: I43e7ea804922de07d153c7e356cf95e2a11fc592
Reviewed-on: https://go-review.googlesource.com/35050
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/build.go

@@ -290,7 +290,8 @@ func defaultContext() Context {
 	// in all releases >= Go 1.x. Code that requires Go 1.x or later should
 	// say "+build go1.x", and code that should only be built before Go 1.x
 	// (perhaps it is the stub to use in that case) should say "+build !go1.x".
-	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7", "go1.8"}
+	// NOTE: If you add to this list, also update the doc comment in doc.go.
+	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5", "go1.6", "go1.7", "go1.8", "go1.9"}
 
 	env := os.Getenv("CGO_ENABLED")
 	if env == "" {

src/go/build/doc.go

@@ -105,6 +105,7 @@
 //	- "go1.6", from Go version 1.6 onward
 //	- "go1.7", from Go version 1.7 onward
 //	- "go1.8", from Go version 1.8 onward
+//	- "go1.9", from Go version 1.9 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // If a file's name, after stripping the extension and a possible _test suffix,