* Compiler changes

* Generated code changes

* Library changes

* Compiler style changes

* Generated style changes

* Fix Windows build errors

* Implement changes from review

* Reintroduce proto2 check

* Compiler changes (required handling review)

* Generated code changes (required handling review)

* Library changes (required handling review

* Field presence rewrite (compiler changes)

* Field presence rewrite (generated code changes)

* Compiler comment

* IFieldAccessor.HasValue library implementation

* Remove Clear methods and default values from proto3 code (Compiler)

* Remove Clear methods and default values from proto3 code (Generated)

* Remove Clear methods and default values from proto3 code (Library)

* Fix distcheck error

* Rewrite default string values to use base64 and convert

* Library changes (IMessage2)

* Compiler changes (IMessage2)

* Generated changes (IMessage2)

* Rebased and regenerated

* Compiler changes (initialized extension)

* Generated changes (initialized extension)

* Library changes (initialized extension)

* Refactor MessageExtensions.IsRequired

* Move string default value creator and bytes default value creator back to seperate methods

* Dead code cleanup

* Fixed segmentation fault
Removed unused header method declarations

Dart build status

fix java build by changing mvn to online

* Adopt upb change for timestamp and duration json to php

* Remove unused code

* Re-sync upb

* Fix php implementation timestamp json parsing

* Fix strptime use local timezone on mac.

* Remove succeeding tests

* Resync

* Add tests for values

* Fix php tests

* Fix encoder handlers change default value

Previously, oneofsubmsg_handler and submsg_handler change zval's default value directly.
The fix use REPLACE_ZVAL_VALUE which create a copy of parsed value and assign it to zval.

Rather than converting the proto to a ByteString again, use the
existing SerializedData property.

This is primarily for access to comments, which would be expected to be available in a protoc plugin.

The implementation has two fiddly aspects:

- We use a Lazy<T> to avoid building the map before cross-linking. An alternative would be to crosslink at the end of the constructor, and remove the calls to CrossLink elsewhere. This would be generally better IMO, but deviate from the Java code.
- The casts to IReadOnlyList<DescriptorBase> are unfortunate. They'll always work, because these lists are always ReadOnlyCollection<T> for a descriptor type... but we can't use IList<DescriptorBase> as that's not covariant, and it's annoyingly fiddly to change the field to be of type ReadOnlyCollection<T>.

Add build status badge for Dart

Merge 3.6.x branch to master

Route "compiler" value of config_setting through @bazel_tools/tools/cpp:compiler.

This performs more testing for field descriptors built from byte
strings too, but that's mostly incidental. The chief intent is to
check that cross-linking occurs.

On Windows invoke plugins using cmd.exe

update to Guava 20

Python protobuf has long been a single-source codebase. 2to3 shouldn't need to run in setup.py.

Upport kokoro/release/python from 3.6.x branch

Down-integrate from google3.

Add experimental collect_all_artifacts script

Update README.md with gradle instructions for java

* Fix conformance running nothing issue

This change adds a source dependency of the test suite implementaion
class in the main function. For generality reason, the main function is
moved to the file of the test suite implemetation.  New test suite
implementation will need to implement the main function.
In order to make it easy for test suite implementation to implement the
main function, this change also refactor out the common code out of the
main function.

* Fix typo

This will find .bat files as well as .exe.

Only affects the case when plugins are invoked from PATH.

This does not change behaviour when compiled under Cygwin..

Added conformance test impl file to CMake.

Fixes JSON Loading of Any messages with Unknown Fields  in C++

Added cmake build files for the conformance test runner.

Add contributing guidelines.