go/build: introduce go1.5 build tag

Change-Id: Iab2f8e1c4443f39b79c1c63a7a30062074b48764
Signed-off-by: Shenghou Ma <minux@golang.org>
Reviewed-on: https://go-review.googlesource.com/10042Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/build/build.go

@@ -296,11 +296,7 @@ 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".
-	//
-	// When we reach Go 1.5 the line will read
-	//	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5"}
-	// and so on.
-	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4"}
+	c.ReleaseTags = []string{"go1.1", "go1.2", "go1.3", "go1.4", "go1.5"}
 
 	switch os.Getenv("CGO_ENABLED") {
 	case "1":

src/go/build/doc.go

@@ -101,6 +101,7 @@
 //	- "go1.2", from Go version 1.2 onward
 //	- "go1.3", from Go version 1.3 onward
 //	- "go1.4", from Go version 1.4 onward
+//	- "go1.5", from Go version 1.5 onward
 //	- any additional words listed in ctxt.BuildTags
 //
 // If a file's name, after stripping the extension and a possible _test suffix,