diff --git a/vendor/google.golang.org/protobuf/internal/impl/legacy_extension.go b/vendor/google.golang.org/protobuf/internal/impl/legacy_extension.go
new file mode 100644
index 0000000..49e7231
--- /dev/null
+++ b/vendor/google.golang.org/protobuf/internal/impl/legacy_extension.go
@@ -0,0 +1,176 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package impl
+
+import (
+	"reflect"
+
+	"google.golang.org/protobuf/internal/descopts"
+	"google.golang.org/protobuf/internal/encoding/messageset"
+	ptag "google.golang.org/protobuf/internal/encoding/tag"
+	"google.golang.org/protobuf/internal/filedesc"
+	"google.golang.org/protobuf/internal/pragma"
+	pref "google.golang.org/protobuf/reflect/protoreflect"
+	preg "google.golang.org/protobuf/reflect/protoregistry"
+	piface "google.golang.org/protobuf/runtime/protoiface"
+)
+
+func (xi *ExtensionInfo) initToLegacy() {
+	xd := xi.desc
+	var parent piface.MessageV1
+	messageName := xd.ContainingMessage().FullName()
+	if mt, _ := preg.GlobalTypes.FindMessageByName(messageName); mt != nil {
+		// Create a new parent message and unwrap it if possible.
+		mv := mt.New().Interface()
+		t := reflect.TypeOf(mv)
+		if mv, ok := mv.(unwrapper); ok {
+			t = reflect.TypeOf(mv.protoUnwrap())
+		}
+
+		// Check whether the message implements the legacy v1 Message interface.
+		mz := reflect.Zero(t).Interface()
+		if mz, ok := mz.(piface.MessageV1); ok {
+			parent = mz
+		}
+	}
+
+	// Determine the v1 extension type, which is unfortunately not the same as
+	// the v2 ExtensionType.GoType.
+	extType := xi.goType
+	switch extType.Kind() {
+	case reflect.Bool, reflect.Int32, reflect.Int64, reflect.Uint32, reflect.Uint64, reflect.Float32, reflect.Float64, reflect.String:
+		extType = reflect.PtrTo(extType) // T -> *T for singular scalar fields
+	}
+
+	// Reconstruct the legacy enum full name.
+	var enumName string
+	if xd.Kind() == pref.EnumKind {
+		enumName = legacyEnumName(xd.Enum())
+	}
+
+	// Derive the proto file that the extension was declared within.
+	var filename string
+	if fd := xd.ParentFile(); fd != nil {
+		filename = fd.Path()
+	}
+
+	// For MessageSet extensions, the name used is the parent message.
+	name := xd.FullName()
+	if messageset.IsMessageSetExtension(xd) {
+		name = name.Parent()
+	}
+
+	xi.ExtendedType = parent
+	xi.ExtensionType = reflect.Zero(extType).Interface()
+	xi.Field = int32(xd.Number())
+	xi.Name = string(name)
+	xi.Tag = ptag.Marshal(xd, enumName)
+	xi.Filename = filename
+}
+
+// initFromLegacy initializes an ExtensionInfo from
+// the contents of the deprecated exported fields of the type.
+func (xi *ExtensionInfo) initFromLegacy() {
+	// The v1 API returns "type incomplete" descriptors where only the
+	// field number is specified. In such a case, use a placeholder.
+	if xi.ExtendedType == nil || xi.ExtensionType == nil {
+		xd := placeholderExtension{
+			name:   pref.FullName(xi.Name),
+			number: pref.FieldNumber(xi.Field),
+		}
+		xi.desc = extensionTypeDescriptor{xd, xi}
+		return
+	}
+
+	// Resolve enum or message dependencies.
+	var ed pref.EnumDescriptor
+	var md pref.MessageDescriptor
+	t := reflect.TypeOf(xi.ExtensionType)
+	isOptional := t.Kind() == reflect.Ptr && t.Elem().Kind() != reflect.Struct
+	isRepeated := t.Kind() == reflect.Slice && t.Elem().Kind() != reflect.Uint8
+	if isOptional || isRepeated {
+		t = t.Elem()
+	}
+	switch v := reflect.Zero(t).Interface().(type) {
+	case pref.Enum:
+		ed = v.Descriptor()
+	case enumV1:
+		ed = LegacyLoadEnumDesc(t)
+	case pref.ProtoMessage:
+		md = v.ProtoReflect().Descriptor()
+	case messageV1:
+		md = LegacyLoadMessageDesc(t)
+	}
+
+	// Derive basic field information from the struct tag.
+	var evs pref.EnumValueDescriptors
+	if ed != nil {
+		evs = ed.Values()
+	}
+	fd := ptag.Unmarshal(xi.Tag, t, evs).(*filedesc.Field)
+
+	// Construct a v2 ExtensionType.
+	xd := &filedesc.Extension{L2: new(filedesc.ExtensionL2)}
+	xd.L0.ParentFile = filedesc.SurrogateProto2
+	xd.L0.FullName = pref.FullName(xi.Name)
+	xd.L1.Number = pref.FieldNumber(xi.Field)
+	xd.L1.Cardinality = fd.L1.Cardinality
+	xd.L1.Kind = fd.L1.Kind
+	xd.L2.IsPacked = fd.L1.IsPacked
+	xd.L2.Default = fd.L1.Default
+	xd.L1.Extendee = Export{}.MessageDescriptorOf(xi.ExtendedType)
+	xd.L2.Enum = ed
+	xd.L2.Message = md
+
+	// Derive real extension field name for MessageSets.
+	if messageset.IsMessageSet(xd.L1.Extendee) && md.FullName() == xd.L0.FullName {
+		xd.L0.FullName = xd.L0.FullName.Append(messageset.ExtensionName)
+	}
+
+	tt := reflect.TypeOf(xi.ExtensionType)
+	if isOptional {
+		tt = tt.Elem()
+	}
+	xi.goType = tt
+	xi.desc = extensionTypeDescriptor{xd, xi}
+}
+
+type placeholderExtension struct {
+	name   pref.FullName
+	number pref.FieldNumber
+}
+
+func (x placeholderExtension) ParentFile() pref.FileDescriptor            { return nil }
+func (x placeholderExtension) Parent() pref.Descriptor                    { return nil }
+func (x placeholderExtension) Index() int                                 { return 0 }
+func (x placeholderExtension) Syntax() pref.Syntax                        { return 0 }
+func (x placeholderExtension) Name() pref.Name                            { return x.name.Name() }
+func (x placeholderExtension) FullName() pref.FullName                    { return x.name }
+func (x placeholderExtension) IsPlaceholder() bool                        { return true }
+func (x placeholderExtension) Options() pref.ProtoMessage                 { return descopts.Field }
+func (x placeholderExtension) Number() pref.FieldNumber                   { return x.number }
+func (x placeholderExtension) Cardinality() pref.Cardinality              { return 0 }
+func (x placeholderExtension) Kind() pref.Kind                            { return 0 }
+func (x placeholderExtension) HasJSONName() bool                          { return false }
+func (x placeholderExtension) JSONName() string                           { return "[" + string(x.name) + "]" }
+func (x placeholderExtension) TextName() string                           { return "[" + string(x.name) + "]" }
+func (x placeholderExtension) HasPresence() bool                          { return false }
+func (x placeholderExtension) HasOptionalKeyword() bool                   { return false }
+func (x placeholderExtension) IsExtension() bool                          { return true }
+func (x placeholderExtension) IsWeak() bool                               { return false }
+func (x placeholderExtension) IsPacked() bool                             { return false }
+func (x placeholderExtension) IsList() bool                               { return false }
+func (x placeholderExtension) IsMap() bool                                { return false }
+func (x placeholderExtension) MapKey() pref.FieldDescriptor               { return nil }
+func (x placeholderExtension) MapValue() pref.FieldDescriptor             { return nil }
+func (x placeholderExtension) HasDefault() bool                           { return false }
+func (x placeholderExtension) Default() pref.Value                        { return pref.Value{} }
+func (x placeholderExtension) DefaultEnumValue() pref.EnumValueDescriptor { return nil }
+func (x placeholderExtension) ContainingOneof() pref.OneofDescriptor      { return nil }
+func (x placeholderExtension) ContainingMessage() pref.MessageDescriptor  { return nil }
+func (x placeholderExtension) Enum() pref.EnumDescriptor                  { return nil }
+func (x placeholderExtension) Message() pref.MessageDescriptor            { return nil }
+func (x placeholderExtension) ProtoType(pref.FieldDescriptor)             { return }
+func (x placeholderExtension) ProtoInternal(pragma.DoNotImplement)        { return }