Code:

package main

import (
    "flag"
    "fmt"
    "testing"

    "github.com/golang/protobuf/jsonpb"
    "github.com/golang/protobuf/proto"

    pb "go.chromium.org/luci/buildbucket/proto"

    . "github.com/smartystreets/goconvey/convey"
    . "go.chromium.org/luci/common/testing/assertions"
)

// special type that implements proto.Message interface for test purpose
type messageImpl string

func (messageImpl) Reset() {}
func (m messageImpl) String() string {
    return string(m)
}
func (messageImpl) ProtoMessage() {}

// Assert that messageImpl implements to the proto.Message interface.
var _ = proto.Message(new(messageImpl))

func TestProtoMessageSliceFlag(t *testing.T) {
    Convey("Basic", t, func() {
        var builds []*pb.Build
        fs := flag.NewFlagSet("test", flag.ContinueOnError)
        flag := MessageSliceFlag(&builds)
        fs.Var(flag, "build", "Test Proto Message")

        var err error
        err = flag.Set("{\"id\": 111111, \"status\": \"SUCCESS\"}")
        So(err, ShouldBeNil)
        err = flag.Set("{\"id\": 222222, \"status\": \"FAILURE\"}")
        So(err, ShouldBeNil)
        err = flag.Set("{\"id\": 333333, \"status\": \"CANCELED\"}")
        So(err, ShouldBeNil)

        So(flag.Get(), ShouldResembleProto, builds)
        So(builds, ShouldHaveLength, 3)
        So(builds[0].Id, ShouldEqual, 111111)
        So(builds[0].Status, ShouldEqual, pb.Status_SUCCESS)
        So(builds[1].Id, ShouldEqual, 222222)
        So(builds[1].Status, ShouldEqual, pb.Status_FAILURE)
        So(builds[2].Id, ShouldEqual, 333333)
        So(builds[2].Status, ShouldEqual, pb.Status_CANCELED)
        // Hard to set expectation for JSON string due to indentation
        // and field order. Therefore, simply check if it's empty or not.
        So(flag.String(), ShouldNotBeBlank)
    })
    Convey("Panic when input is not pointer", t, func() {
        So(func() { MessageSliceFlag("a string") }, ShouldPanic)
    })
    Convey("Panic when input pointer dose not point to a slice", t, func() {
        So(func() { MessageSliceFlag(&map[string]string{}) }, ShouldPanic)
    })
    Convey("Panic when element does not implement proto.Message", t, func() {
        So(func() { MessageSliceFlag(&[]string{}) }, ShouldPanic)
    })
    Convey("Panic when element is not pointer", t, func() {
        So(func() { MessageSliceFlag(&[]messageImpl{}) }, ShouldPanic)
    })
    Convey("Panic when element pointer does not point to a struct", t, func() {
        So(func() { MessageSliceFlag(&[]*messageImpl{}) }, ShouldPanic)
    })
}

func main() {
    var builds []*pb.Build
    fs := flag.NewFlagSet("test", flag.ContinueOnError)
    flag := MessageSliceFlag(&builds)
    fs.Var(flag, "build", "Test Proto Message")
    fs.Parse([]string{"-build",
        "{\"id\": 111111, \"status\": \"SUCCESS\"}"})
    marshaler := &jsonpb.Marshaler{Indent: "  "}
    jsonMsg, _ := marshaler.MarshalToString(builds[0])
    fmt.Println(jsonMsg)
}