Add missing export annotation for BinaryReader.readStringRequireUtf8

BinaryReader.readStringRequireUtf8 may be called by users
(particularly the protobuf conformance suite) and therefore needs to
visible after minification/google closure prop renaming.

This change also fixes a type warning around something we assert is
present.

Author: dibenede

binary/decoder.js

@@ -902,7 +902,8 @@ jspb.BinaryDecoder.prototype.readString = function (length, requireUtf8) {
   this.cursor_ += length;
   this.checkCursor();
   const result =
-	jspb.binary.utf8.decodeUtf8(jspb.asserts.assert(this.bytes_), cursor, length, requireUtf8);
+    jspb.binary.utf8.decodeUtf8(/** @type {!Uint8Array} */(jspb.asserts.assert(this.bytes_)),
+      cursor, length, requireUtf8);
   return result;
 };
 

binary/reader.js

@@ -1034,6 +1034,7 @@ jspb.BinaryReader.prototype.readString = function() {
  * not valid utf8.
  *
  * @return {string} The value of the string field.
+ * @export
  */
 jspb.BinaryReader.prototype.readStringRequireUtf8 = function () {
   jspb.asserts.assert(this.nextWireType_ == jspb.BinaryConstants.WireType.DELIMITED);