net/http: remove init func reference to ServeMux

Shrinks cmd/go by 30KB.

Change-Id: Ied31192e85af76ebac743f8cc12bd9ef6ec5048f
Reviewed-on: https://go-review.googlesource.com/20765Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/http_test.go

@@ -86,6 +86,8 @@ func TestCmdGoNoHTTPServer(t *testing.T) {
 		// Verify these don't exist:
 		"net/http.http2Server":           false,
 		"net/http.(*Server).Serve":       false,
+		"net/http.(*ServeMux).ServeHTTP": false,
+		"net/http.DefaultServeMux":       false,
 	}
 	for sym, want := range wantSym {
 		got := bytes.Contains(out, []byte(sym))

src/net/http/server.go

@@ -1800,10 +1800,12 @@ type muxEntry struct {
 }
 
 // NewServeMux allocates and returns a new ServeMux.
-func NewServeMux() *ServeMux { return &ServeMux{m: make(map[string]muxEntry)} }
+func NewServeMux() *ServeMux { return new(ServeMux) }
 
 // DefaultServeMux is the default ServeMux used by Serve.
-var DefaultServeMux = NewServeMux()
+var DefaultServeMux = &defaultServeMux
+
+var defaultServeMux ServeMux
 
 // Does path match pattern?
 func pathMatch(pattern, path string) bool {
@@ -1926,6 +1928,9 @@ func (mux *ServeMux) Handle(pattern string, handler Handler) {
 		panic("http: multiple registrations for " + pattern)
 	}
 
+	if mux.m == nil {
+		mux.m = make(map[string]muxEntry)
+	}
 	mux.m[pattern] = muxEntry{explicit: true, h: handler, pattern: pattern}
 
 	if pattern[0] != '/' {