Allow multiple dart <script> tags in .sky files

sky/engine/core/BUILD.gn

Index: sky/engine/core/BUILD.gn
diff --git a/sky/engine/core/BUILD.gn b/sky/engine/core/BUILD.gn
index b7daba1b5b7f94e4511b0be205193a04ecc7ca3d..d01e064df5341da09355a39f9bdb8799dad0b686 100644
--- a/sky/engine/core/BUILD.gn
+++ b/sky/engine/core/BUILD.gn
@@ -66,6 +66,11 @@ static_library("core") {
 
   sources = sky_core_files
 
+  include_dirs = [
+    # Needed for dart_mirrors_api.h in dart_controller.cc
+    rebase_path("//dart/runtime"),
+  ]
+
   forward_dependent_configs_from = [ ":libraries" ]
 
   # core and core_generated are really the same thing.