go/build: add go1.10 build tag

Add this early in the cycle so that we can start regression testing
of the master toolchain.

Change-Id: Ida3ccad6e9642648f489babd12877fc8a5eca07a
Reviewed-on: https://go-review.googlesource.com/59151Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/go/build/build.go

@@ -292,7 +292,10 @@ func defaultContext() Context {
 	// 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".
 	// 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"}
+	const version = 10 // go1.10
+	for i := 1; i <= version; i++ {
+		c.ReleaseTags = append(c.ReleaseTags, "go1."+strconv.Itoa(i))
+	}
 
 	env := os.Getenv("CGO_ENABLED")
 	if env == "" {

src/go/build/doc.go

@@ -106,6 +106,7 @@
 //	- "go1.7", from Go version 1.7 onward
 //	- "go1.8", from Go version 1.8 onward
 //	- "go1.9", from Go version 1.9 onward
+//	- "go1.10", from Go version 1.10 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // If a file's name, after stripping the extension and a possible _test suffix,