Follow ObjC conventions and how the generated header labels things by
copying NSStrings/NSData fields when setting them.

Causes a warning in newer Xcodes.

Added VSCode extension link

Added link to VSCode-Proto3 extension in Other Utilities section

Basic Proto2 support for Ruby gem

This change only adds basic proto2 support without advanced features
like extensions, custom options, maps, etc.

The protoc binary now generates ruby code for proto2 syntax.
However, for now, it is restricted to proto2 files without advanced features
like extensions, in which case it still errors out.

This change also modifies the DSL to add proto messages to the DescriptorPool.
There is a new DSL Builder#add_file to create a new FileDescriptor. With this,
the generated ruby DSL looks something like:

Google::Protobuf::DescriptorPool.generated_pool.build do
  add_file "test.proto" do
    add_message "foo" do
      optional :val, :int32, 1
    end
  end
end

Smear comments

* 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

Fixes #1890.

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