Remove "using namespace std" from stubs/common.h

Remove unused constants.

This prevents the contents of the std namespace from being effectively
pulled into the top-level namespace in all translation units that
include common.h. I left in individual using statements for a few common
things like std::set and std::map, because it did not seem worth going
through the churn of updating the whole codebase to fix those right now.

When compiling with -Werror, -Wunused-const-variable the build fails due
to those two constants not being used.

Add a link to dart-lang/protobuf

Add gogoprotobuf to third-party add-ons list

add a key to ctx.action dict to prevent protoc losing the default env

Add node-protoc-plugin to "Other Utilities"

Simplify the Element dtor invocation when freeing elements in InternalDeallocate

Tighten up class usage/checks.

- Ensure extensions resolution/wiring is happening directly on the
  messageClass (incase someone is doing odd things our out classes).
- Make the extension message check match the other class checks in
  for mergeFrom/isEqual/etc.

InternalDeallocate to avoid confusing the compiler when there's
a class named Element already defined in the global namespace.

Allow unknown values for Map put*Value methods just like every other …

Fix duplicate fields test and Any test. The repeated version is passing because nu…

Use PyUnicode_AsEncodedString() instead of PyUnicode_AsEncodedObject()

Fix test for unexpected type url when parsing Any. Currently, the test fails since TestAllTypes doesn't have field '@type', which is the same test as testUnknownFields.

Adding default shell env to allow non-default compilers

Fix error message for int64 parse failure.

Fix duplicate fields test. The repeated version is passing because null values in a repeated field are rejected and not testing what it wanted to. Also adds a oneof version that verifies the case of oneof fields of different names (currently only same name check seems to be tested).

Also fix spelling of a test.

JS compatibility tests

I am adding default shell environment, so that protobuf compilation makes use of set
LD_LIBRARY_PATH and so on.
In this way, also non-default gcc installations (e.g. not in /usr/lib) can be used to compile protobuf.
This would fix the following issue:
https://github.com/bazelbuild/bazel/issues/2515

See https://github.com/google/protobuf/issues/2425

Remove the use of dispatch_once that is heap backed.

Apple recently updated the docs on dispatch_once to point out
that the storage for the dispatch_once_t must be static or global,
but not something that was ever used before as the implementation
doesn't use a memory barrier.  So we drop the use and create the
semaphore when needed and use an atomic swap deal with any
threading races.

Add support for Xcode 8.3 to the build helper.

Add some new ignores for things generated in conformance.

* Issue #2211: Addressing GCC warning on enumeral/non-enumeral in conditional expression.

* Updated per 80 character wrap.

Added a workaround to allow building for NaCl

arm-nacl-clang seems to produce a mysterious compiler error when it
encounters __builtin_clzll, so when we are building for NaCl, let's
avoid that in favor of the portable implementation of
Log2FloorNonZero64.