AssertionError: Assertion failed when deserializing message from python in js

What version of protobuf and what language are you using?
Version: v.3.11.2
Language: Python, Javascript

What operating system (Linux, Windows, ...) and version?
MacOS High Sierra Version 10.13.6

What runtime / compiler are you using (e.g., python version or gcc version)
Python 3.7.3, NodeJS v13.3.0

What did you do?
Steps to reproduce the behavior:

1. Defined these protos:

syntax = "proto2";

message B {
  optional string b2 = 2;
}

message A {
  optional B b = 1;
  optional string id = 2;
}

2. Attempted to deserialize this message in both js and python: 0a060a0131120131181e10a3c6ad8dcdeed0a274

Here's a sample repo to reproduce: https://github.com/bhainesva/protobuf-py-vs-js-demo (run node test.js / python3 test.py after installing requirements).

What did you expect to see
I expected the deserialization to succeed in both python and js.

What did you see instead?
Python deserialization succeeds but js deserialization produces this error:

/Users/bhaines/repo/proto-demo/node_modules/google-protobuf/google-protobuf.js:81
jspb.BinaryConstants.TWO_TO_32=4294967296;jspb.BinaryConstants.TWO_TO_52=4503599627370496;jspb.BinaryConstants.TWO_TO_63=0x7fffffffffffffff;jspb.BinaryConstants.TWO_TO_64=1.8446744073709552E19;jspb.BinaryConstants.ZERO_HASH="\x00\x00\x00\x00\x00\x00\x00\x00";goog.dom={};goog.dom.NodeType={ELEMENT:1,ATTRIBUTE:2,TEXT:3,CDATA_SECTION:4,ENTITY_REFERENCE:5,ENTITY:6,PROCESSING_INSTRUCTION:7,COMMENT:8,DOCUMENT:9,DOCUMENT_TYPE:10,DOCUMENT_FRAGMENT:11,NOTATION:12};goog.debug={};goog.debug.Error=function(a){if(Error.captureStackTrace)Error.captureStackTrace(this,goog.debug.Error);else{var b=Error().stack;b&&(this.stack=b)}a&&(this.message=String(a));this.reportErrorToServer=!0};goog.inherits(goog.debug.Error,Error);goog.debug.Error.prototype.name="CustomError";goog.asserts={};goog.asserts.ENABLE_ASSERTS=goog.DEBUG;goog.asserts.AssertionError=function(a,b){goog.debug.Error.call(this,goog.asserts.subs_(a,b));this.messagePattern=a};goog.in
Error [AssertionError]: Assertion failed
    at new goog.asserts.AssertionError (/Users/bhaines/repo/proto-demo/node_modules/google-protobuf/google-protobuf.js:81:876)
    at Object.goog.asserts.doAssertFailure_ (/Users/bhaines/repo/proto-demo/node_modules/google-protobuf/google-protobuf.js:82:257)
    at Object.goog.asserts.assert (/Users/bhaines/repo/proto-demo/node_modules/google-protobuf/google-protobuf.js:83:83)
    at jspb.BinaryReader.readString (/Users/bhaines/repo/proto-demo/node_modules/google-protobuf/google-protobuf.js:394:64)
    at Function.proto.A.deserializeBinaryFromReader (/Users/bhaines/repo/proto-demo/demo_pb.js:263:49)
    at Function.proto.A.deserializeBinary (/Users/bhaines/repo/proto-demo/demo_pb.js:239:18)
    at Object.<anonymous> (/Users/bhaines/repo/proto-demo/test.js:13:20)
    at Module._compile (internal/modules/cjs/loader.js:1128:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1167:10)
    at Module.load (internal/modules/cjs/loader.js:983:32) {
  message: 'Assertion failed',
  reportErrorToServer: true,
  messagePattern: 'Assertion failed'
}

Seems related to including unknown fields in messages but I'm not very familiar with protobuf, my understanding is that that's supported.

Potentially related to this issue? protocolbuffers/protobuf#4688.

Start with the linked sample repository and run node test.js and python3 test.py after installing its requirements. Compare the deserialization path in demo_pb.js with the Python result, using the reported byte sequence and stack trace as the reproduction. Done means the message deserializes successfully in both JavaScript and Python without the AssertionError.