Prepare a standalone non-interative Sky shell on Mac. (#2700)

sky/dist/BUILD.gn

@@ -22,7 +22,7 @@ copy("sky_shell") {
     ]
 
     deps = [
-      "//sky/shell:shell_struct",
+      "//sky/shell",
     ]
   } else if (!is_ios) {
     sources = [

sky/shell/BUILD.gn

@@ -463,7 +463,14 @@ if (is_android) {
     ]
   }
 
-  mac_app("shell") {
+  executable("shell_standalone") {
+    output_name = "sky_shell"
+    deps = [
+      ":mac_scaffolding",
+    ]
+  }
+
+  mac_app("shell_application") {
     app_name = "SkyShell"
     info_plist = "platform/mac/Info.plist"
 
@@ -481,6 +488,13 @@ if (is_android) {
       ":sky_resources",
     ]
   }
+
+  group("shell") {
+    deps = [
+      ":shell_application",
+      ":shell_standalone",
+    ]
+  }
 } else {
   assert(false, "Unsupported platform")
 }