func Get(name string) Builder {
	if b, ok := m[name]; ok {
		return b
	}
	return defaultBuilder
}
Get returns the resolver builder registered with the given name.
If no builder is register with the name, the default pickfirst will
be used.
package balancer
import (
	"errors"
	"net"
	"golang.org/x/net/context"
	"google.golang.org/grpc/connectivity"
	"google.golang.org/grpc/credentials"
	"google.golang.org/grpc/resolver"
)
var (
	m = make(map[string]Builder)
	defaultBuilder Builder  
)
func Register(b Builder) {
	m[b.Name()] = b
}