diff --git a/vendor/google.golang.org/protobuf/internal/encoding/json/decode_string.go b/vendor/google.golang.org/protobuf/internal/encoding/json/decode_string.go
new file mode 100644
index 0000000..f7fea7d
--- /dev/null
+++ b/vendor/google.golang.org/protobuf/internal/encoding/json/decode_string.go
@@ -0,0 +1,91 @@
+// 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 json
+
+import (
+	"strconv"
+	"unicode"
+	"unicode/utf16"
+	"unicode/utf8"
+
+	"google.golang.org/protobuf/internal/strs"
+)
+
+func (d *Decoder) parseString(in []byte) (string, int, error) {
+	in0 := in
+	if len(in) == 0 {
+		return "", 0, ErrUnexpectedEOF
+	}
+	if in[0] != '"' {
+		return "", 0, d.newSyntaxError(d.currPos(), "invalid character %q at start of string", in[0])
+	}
+	in = in[1:]
+	i := indexNeedEscapeInBytes(in)
+	in, out := in[i:], in[:i:i] // set cap to prevent mutations
+	for len(in) > 0 {
+		switch r, n := utf8.DecodeRune(in); {
+		case r == utf8.RuneError && n == 1:
+			return "", 0, d.newSyntaxError(d.currPos(), "invalid UTF-8 in string")
+		case r < ' ':
+			return "", 0, d.newSyntaxError(d.currPos(), "invalid character %q in string", r)
+		case r == '"':
+			in = in[1:]
+			n := len(in0) - len(in)
+			return string(out), n, nil
+		case r == '\\':
+			if len(in) < 2 {
+				return "", 0, ErrUnexpectedEOF
+			}
+			switch r := in[1]; r {
+			case '"', '\\', '/':
+				in, out = in[2:], append(out, r)
+			case 'b':
+				in, out = in[2:], append(out, '\b')
+			case 'f':
+				in, out = in[2:], append(out, '\f')
+			case 'n':
+				in, out = in[2:], append(out, '\n')
+			case 'r':
+				in, out = in[2:], append(out, '\r')
+			case 't':
+				in, out = in[2:], append(out, '\t')
+			case 'u':
+				if len(in) < 6 {
+					return "", 0, ErrUnexpectedEOF
+				}
+				v, err := strconv.ParseUint(string(in[2:6]), 16, 16)
+				if err != nil {
+					return "", 0, d.newSyntaxError(d.currPos(), "invalid escape code %q in string", in[:6])
+				}
+				in = in[6:]
+
+				r := rune(v)
+				if utf16.IsSurrogate(r) {
+					if len(in) < 6 {
+						return "", 0, ErrUnexpectedEOF
+					}
+					v, err := strconv.ParseUint(string(in[2:6]), 16, 16)
+					r = utf16.DecodeRune(r, rune(v))
+					if in[0] != '\\' || in[1] != 'u' ||
+						r == unicode.ReplacementChar || err != nil {
+						return "", 0, d.newSyntaxError(d.currPos(), "invalid escape code %q in string", in[:6])
+					}
+					in = in[6:]
+				}
+				out = append(out, string(r)...)
+			default:
+				return "", 0, d.newSyntaxError(d.currPos(), "invalid escape code %q in string", in[:2])
+			}
+		default:
+			i := indexNeedEscapeInBytes(in[n:])
+			in, out = in[n+i:], append(out, in[:n+i]...)
+		}
+	}
+	return "", 0, ErrUnexpectedEOF
+}
+
+// indexNeedEscapeInBytes returns the index of the character that needs
+// escaping. If no characters need escaping, this returns the input length.
+func indexNeedEscapeInBytes(b []byte) int { return indexNeedEscapeInString(strs.UnsafeString(b)) }