Make passthrouth resolver the default instead of dns (#1606)

resolver/passthrough/passthrough.go

@@ -17,8 +17,7 @@
  */
 
 // Package passthrough implements a pass-through resolver. It sends the target
-// name without scheme back to gRPC as resolved address. It's for gRPC internal
-// test only.
+// name without scheme back to gRPC as resolved address.
 package passthrough
 
 import "google.golang.org/grpc/resolver"

resolver/resolver.go

@@ -24,7 +24,7 @@ var (
 	// m is a map from scheme to resolver builder.
 	m = make(map[string]Builder)
 	// defaultScheme is the default scheme to use.
-	defaultScheme = "dns"
+	defaultScheme = "passthrough"
 )
 
 // TODO(bar) install dns resolver in init(){}.