Use tree artifacts in bootclasspath rule

Remote execution now creates the directories for output tree artifacts
prior to execution, so that the output of the Java compiler can be
modelled as a tree artifact even though not all versions create the
output directory if it doesn't exist.

tools/jdk/default_java_toolchain.bzl

@@ -183,16 +183,7 @@ def java_runtime_files(name, srcs):
 def _bootclasspath_impl(ctx):
     host_javabase = ctx.attr.host_javabase[java_common.JavaRuntimeInfo]
 
-    # explicitly list output files instead of using TreeArtifact to work around
-    # https://github.com/bazelbuild/bazel/issues/6203
-    classes = [
-        "DumpPlatformClassPath.class",
-    ]
-
-    class_outputs = [
-        ctx.actions.declare_file("%s_classes/%s" % (ctx.label.name, clazz))
-        for clazz in classes
-    ]
+    class_dir = ctx.actions.declare_directory("%s_classes" % ctx.label.name)
 
     args = ctx.actions.args()
     args.add("-source")
@@ -203,20 +194,20 @@ def _bootclasspath_impl(ctx):
     args.add("-cp")
     args.add("%s/lib/tools.jar" % host_javabase.java_home)
     args.add("-d")
-    args.add(class_outputs[0].dirname)
+    args.add_all([class_dir], expand_directories = False)
     args.add(ctx.file.src)
 
     ctx.actions.run(
         executable = "%s/bin/javac" % host_javabase.java_home,
         mnemonic = "JavaToolchainCompileClasses",
         inputs = [ctx.file.src] + ctx.files.host_javabase,
-        outputs = class_outputs,
+        outputs = [class_dir],
         arguments = [args],
     )
 
     bootclasspath = ctx.outputs.output_jar
 
-    inputs = class_outputs + ctx.files.host_javabase
+    inputs = [class_dir] + ctx.files.host_javabase
 
     args = ctx.actions.args()
     args.add("-XX:+IgnoreUnrecognizedVMOptions")
@@ -225,8 +216,9 @@ def _bootclasspath_impl(ctx):
     args.add("--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED")
     args.add_joined(
         "-cp",
-        [class_outputs[0].dirname, "%s/lib/tools.jar" % host_javabase.java_home],
+        [class_dir, "%s/lib/tools.jar" % host_javabase.java_home],
         join_with = ctx.configuration.host_path_separator,
+        expand_directories = False,
     )
     args.add("DumpPlatformClassPath")
     args.add(bootclasspath)