Add templates to generate fuchsia host bundles (flutter/engine#13158)

This is part of an effort to separate generation of
host artifacts and target artifacts for fuchsia. The
`fuchsia_host_bundle` template aims to capture all the
artifacts that are specific to a given host.

The next step would be to bundle these are separate CIPD
packages for mac and linux (only x64 hosts)

internal planning doc: go/flutter-fuchsia-packaging

engine/src/flutter/shell/platform/fuchsia/BUILD.gn

@@ -4,25 +4,75 @@
 
 import("//build/fuchsia/sdk.gni")
 import("$flutter_root/common/config.gni")
+import("$flutter_root/tools/fuchsia/dart.gni")
+import("$flutter_root/tools/fuchsia/fuchsia_host_bundle.gni")
 
 if (using_fuchsia_sdk) {
   testonly = true
 
   product_suffix = ""
+  is_product = false
 
   if (flutter_runtime_mode == "release") {
     product_suffix = "product_"
+    is_product = true
+  }
+
+  fuchsia_host_bundle("flutter_binaries") {
+    name = "flutter_binaries"
+
+    _flutter_tester_label = "//flutter/shell/testing:testing($host_toolchain)"
+
+    deps = [
+      _flutter_tester_label,
+    ]
+
+    _flutter_tester_bin_path =
+        rebase_path(get_label_info(_flutter_tester_label, "root_out_dir") +
+                    "/flutter_tester")
+
+    sources = [
+      _flutter_tester_bin_path,
+    ]
+  }
+
+  fuchsia_host_bundle("dart_binaries") {
+    name = "dart_binaries"
+
+    _gen_snapshot_to_use = gen_snapshot + "($host_toolchain)"
+    if (is_product) {
+      _gen_snapshot_to_use = gen_snapshot_product + "($host_toolchain)"
+    }
+
+    _kernel_compiler_label = "dart:kernel_compiler($host_toolchain)"
+
+    deps = [
+      _gen_snapshot_to_use,
+      _kernel_compiler_label,
+    ]
+
+    _gen_snapshot_bin_path =
+        rebase_path(get_label_info(_gen_snapshot_to_use, "root_out_dir") + "/" +
+                    get_label_info(_gen_snapshot_to_use, "name"))
+
+    _kernel_compiler_path =
+        rebase_path(get_label_info(_kernel_compiler_label, "root_gen_dir") +
+                    "/kernel_compiler.dart.snapshot")
+
+    sources = [
+      _gen_snapshot_bin_path,
+      _kernel_compiler_path,
+    ]
   }
 
   group("fuchsia") {
     deps = [
-      "dart:kernel_compiler",
+      ":dart_binaries",
+      ":flutter_binaries",
       "dart_runner:dart_aot_${product_suffix}runner",
       "dart_runner:dart_jit_${product_suffix}runner",
       "flutter:flutter_aot_${product_suffix}runner",
       "flutter:flutter_jit_${product_suffix}runner",
-      "flutter:flutter_runner_tests",
-      "//flutter/shell/testing:testing($host_toolchain)",
     ]
   }
 }

engine/src/flutter/shell/testing/BUILD.gn

@@ -5,8 +5,6 @@
 import("//build/fuchsia/sdk.gni")
 
 executable("testing") {
-  testonly = true
-
   output_name = "flutter_tester"
 
   public_configs = [

engine/src/flutter/tools/fuchsia/fuchsia_host_bundle.gni

@@ -0,0 +1,20 @@
+# Copyright 2013 The Flutter Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+template("fuchsia_host_bundle") {
+  assert(defined(invoker.name), "'name' must be defined for $target_name.")
+  dest_dir = "${root_out_dir}/host_bundle/${invoker.name}-${host_os}-${host_cpu}"
+
+  copy(target_name) {
+    if (defined(invoker.deps)) {
+      deps = invoker.deps
+    }
+
+    sources = invoker.sources
+
+    outputs = [
+      "$dest_dir/{{source_file_part}}",
+    ]
+  }
+}