Build Windows shell (flutter/engine#8331)

Enables the build of the Windows shell, based on the same GLFW code used
by the current Linux shell.

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

@@ -13,9 +13,12 @@ group("platform") {
     ]
   } else if (is_linux) {
     deps = []
-  } else if (is_win || is_fuchsia) {
-    # There is no platform target on Windows. Fuchsia has its own runner
-    # implementation.
+  } else if (is_win) {
+    deps = [
+      "windows",
+    ]
+  } else if ( is_fuchsia) {
+    # Fuchsia has its own runner implementation.
     deps = []
   } else {
     assert(false, "Unknown/Unsupported platform.")

engine/src/flutter/shell/platform/common/cpp/client_wrapper/publish.gni

@@ -23,7 +23,10 @@ template("publish_client_wrapper") {
   }
 
   copy("${template_target_name}_publish_includes") {
-    visibility = [ ":$template_target_name" ]
+    visibility = [
+      ":$template_target_name",
+      ":${template_target_name}_publish_sources",
+    ]
 
     sources = invoker.public
     outputs = [
@@ -38,5 +41,12 @@ template("publish_client_wrapper") {
     outputs = [
       "$publish_dir_root/{{source_file_part}}",
     ]
+
+    # GN on Windows appears to do #include checks even for copy
+    # targets, so add the dependency to the headers to satisfy
+    # the check.
+    deps = [
+      ":${template_target_name}_publish_includes",
+    ]
   }
 }

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

@@ -0,0 +1,22 @@
+# 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.
+
+assert(is_win)
+
+group("windows") {
+  deps = [
+    ":flutter_windows",
+    "$flutter_root/shell/platform/glfw:publish_headers_glfw",
+    "$flutter_root/shell/platform/common/cpp/client_wrapper:publish_wrapper",
+    "$flutter_root/shell/platform/glfw/client_wrapper:publish_wrapper_glfw",
+  ]
+}
+
+shared_library("flutter_windows") {
+  deps = [
+    "$flutter_root/shell/platform/glfw:flutter_glfw",
+  ]
+
+  public_configs = [ "$flutter_root:config" ]
+}