[fuchsia] Add build rules for flags.

Change-Id: Ieccb6df61e5ff67c3416ab53fdfe908ad18e0779
diff --git a/absl/BUILD.gn b/absl/BUILD.gn
index 31344d2..7120502 100644
--- a/absl/BUILD.gn
+++ b/absl/BUILD.gn
@@ -13,5 +13,6 @@
 
   public_deps = [
     "//third_party/abseil-cpp/absl/strings",
+    "//third_party/abseil-cpp/absl/flags",
   ]
 }
diff --git a/absl/flags/BUILD.gn b/absl/flags/BUILD.gn
new file mode 100644
index 0000000..f0e7ac4
--- /dev/null
+++ b/absl/flags/BUILD.gn
@@ -0,0 +1,75 @@
+# Copyright 2019 The Fuchsia Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Please do not add more entries to this list. We plan to remove
+# abseil-cpp from the Fuchsia platform source tree once cobalt
+# moves out-of-tree.
+visibility = [
+  "//src/ledger/*",
+  "//third_party/abseil-cpp/*",
+  "$cobalt_root/*",
+  "//third_party/tink/*",
+]
+
+static_library("flags") {
+  public_configs = [ "//third_party/abseil-cpp:abseil_config" ]
+
+  sources = [
+    "config.h",
+    "declare.h",
+    "flag.cc",
+    "flag.h",
+    "marshalling.cc",
+    "marshalling.h",
+    "parse.cc",
+    "parse.h",
+    "usage.cc",
+    "usage_config.cc",
+    "usage_config.h",
+    "usage.h",
+  ]
+
+  public_deps = [
+    "../strings",
+    "../synchronization",
+  ]
+
+  deps = [
+    ":internal",
+    "../base",
+    "../base:config",
+    "../base:core_headers",
+    "../base:endian",
+    "../base:throw_delegate",
+    "../memory",
+    "../meta:type_traits",
+    "../numeric:int128",
+  ]
+}
+
+static_library("internal") {
+  public_configs = [ "//third_party/abseil-cpp:abseil_config" ]
+
+  public_deps = [
+    "../strings",
+    "../synchronization",
+  ]
+
+  sources = [
+    "internal/commandlineflag.cc",
+    "internal/commandlineflag.h",
+    "internal/flag.cc",
+    "internal/flag.h",
+    "internal/parse.h",
+    "internal/path_util.h",
+    "internal/program_name.cc",
+    "internal/program_name.h",
+    "internal/registry.cc",
+    "internal/registry.h",
+    "internal/type_erased.cc",
+    "internal/type_erased.h",
+    "internal/usage.cc",
+    "internal/usage.h",
+  ]
+}