Add a platform-abstraction tidy script

This is intended to maintain existing standards of code organization
in hopes that the standard library will continue to be refactored to
isolate platform-specific bits, making porting easier; where "standard
library" roughly means "all the dependencies of the std and test
crates".

This generally means placing restrictions on where `cfg(unix)`,
`cfg(windows)`, `cfg(target_os)` and `cfg(target_env)` may appear,
the basic objective being to isolate platform-specific code to the
platform-specific `std::sys` modules, and to the allocation,
unwinding, and libc crates.

Following are the basic rules, though there are currently
exceptions:

- core may not have platform-specific code
- liballoc_system may have platform-specific code
- liballoc_jemalloc may have platform-specific code
- libpanic_abort may have platform-specific code
- libpanic_unwind may have platform-specific code
- other crates in the std facade may not
- std may have platform-specific code in the following places
  - sys/unix/
  - sys/windows/
  - os/

There are plenty of exceptions today though, noted in the whitelist.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/libstd/sync/mpsc/select.rs b/src/libstd/sync/mpsc/select.rs
index 91896e1ab85..e056a350815 100644
--- a/src/libstd/sync/mpsc/select.rs
+++ b/src/libstd/sync/mpsc/select.rs
@@ -366,8 +366,8 @@ impl<'rx, T:Send+'rx> fmt::Debug for Handle<'rx, T> {
     }
 }
 
-#[cfg(all(test, not(target_os = "emscripten")))]
 #[allow(unused_imports)]
+#[cfg(all(test, not(target_os = "emscripten")))]
 mod tests {
     use thread;
     use sync::mpsc::*;