Log Message:
Update go to 1.5.

This release now needs the previous one (lang/go14) to build.

The biggest developments in the implementation are:

* The compiler and runtime are now written entirely in Go (with a little
  assembler). C is no longer involved in the implementation, and so the
  C compiler that was once necessary for building the distribution is
  gone.
* The garbage collector is now concurrent and provides dramatically
  lower pause times by running, when possible, in parallel with other
  goroutines.
* By default, Go programs run with GOMAXPROCS set to the number of cores
  available; in prior releases it defaulted to 1.
* Support for internal packages is now provided for all repositories,
  not just the Go core.
* The go command now provides experimental support for "vendoring"
  external dependencies.
* A new go tool trace command supports fine-grained tracing of program
  execution.
* A new go doc command (distinct from godoc) is customized for
  command-line use.

Full release notes are at https://golang.org/doc/go1.5.