Fix bazel_go_example_test. Changes:

* The go_prefix should be declared in the toplevel BUILD file as
  //:go_prefix, rather than //external:go_prefix.

* Run set_up from the test methods.

* Update references to Darwin.

Tested:
  bazel test src/test/shell/bazel/bazel_go_example_test

--
Change-Id: I89b2f5920100d27abdbc093437c7c051a24fad1e
Reviewed-on: https://bazel-review.googlesource.com/#/c/2561
MOS_MIGRATED_REVID=110762730

Author: hanwen

README.md

@@ -21,7 +21,6 @@ The rules should be considered experimental. They support:
 
 They currently do not support (in order of importance):
 
-* Darwin
 * `//+build` tags
 * auto generated BUILD files.
 * C/C++ interoperation (cgo, swig etc.)
@@ -70,7 +69,7 @@ They currently do not support (in order of importance):
 
 * In this case, import the library as `github.com/joe/project/lib`.
 * For vendored libraries, you may depend on
-  `//lib/vendor/github.com/user/project:go_default_library`. Vendored
+  `//lib/vendor/github.com/user/project:go_default_library`. Vendored
   libraries should have BUILD files like normal libraries.
 * To declare a test,
 

def.bzl

@@ -334,7 +334,7 @@ go_library_attrs = {
     ),
     "go_prefix": attr.label(
         providers = ["go_prefix"],
-        default = Label("//external:go_prefix"),
+        default = Label("//:go_prefix"),
         allow_files = False,
         cfg = HOST_CFG,
     ),
@@ -380,12 +380,7 @@ go_test = rule(
     test = True,
 )
 
-# TODO(bazel-team): support darwin
 def go_repositories():
-  native.bind(name  = "go_prefix",
-    actual = "//:go_prefix",
-  )
-
   native.new_http_archive(
     name=  "golang-linux-amd64",
     url = "https://storage.googleapis.com/golang/go1.5.1.linux-amd64.tar.gz",