Added alias for extdir in tools/jdk.

Some projects fail [1] to build after the change that renamed extdir to extclasspath [2].

[1] http://ci.bazel.io/job/rules_appengine/BAZEL_VERSION=HEAD,PLATFORM_NAME=linux-x86_64/564/console
[2] 8153cb80f4a4424640e694cb6ed4792dcc193171

--
PiperOrigin-RevId: 148085503
MOS_MIGRATED_REVID=148085503
diff --git a/tools/jdk/BUILD b/tools/jdk/BUILD
index 0bd8db3..cbfe203 100644
--- a/tools/jdk/BUILD
+++ b/tools/jdk/BUILD
@@ -81,6 +81,12 @@
     actual = "@local_jdk//:extdir",
 )
 
+# TODO(cushon): migrate to extclasspath and delete
+alias(
+    name = "extdir",
+    actual = "@local_jdk//:extdir",
+)
+
 filegroup(
     name = "langtools",
     srcs = ["//third_party/java/jdk/langtools:javac_jar"],