Correct libraries.yaml path for stub_ui, add brightness, copy dart2js_platform.dill files (flutter/engine#7725)

engine/src/flutter/BUILD.gn

@@ -36,6 +36,7 @@ group("flutter") {
         "$flutter_root/frontend_server",
         "//third_party/dart:create_sdk",
         "$flutter_root/lib/stub_ui:stub_ui",
+        ":dart2js_platform_files"
       ]
       foreach(snapshot, _flutter_sdk_snapshots) {
         public_deps += [ ":copy_flutter_${snapshot[0]}_snapshot" ]
@@ -91,3 +92,18 @@ foreach(snapshot, _flutter_sdk_snapshots) {
     ]
   }
 }
+
+copy("dart2js_platform_files") {
+  deps = [
+    "//third_party/dart/utils/compiler:compile_dart2js_platform"
+  ]
+
+  sources = [
+    "$root_out_dir/dart2js_outline.dill",
+    "$root_out_dir/dart2js_platform.dill",
+  ]
+
+  outputs = [
+    "$root_out_dir/flutter_patched_sdk/{{source_file_part}}",
+  ]
+}