appveyor: verify that js/ts files are listed in BUILD.gn

.appveyor.yml

@@ -377,6 +377,17 @@ after_test:
         throw "Build should be up-to-date but isn't."
       }
 
+  # Verify that javascript and typescript files which are bundled by rollup are
+  # listed explicitly in BUILD.gn. This is not an air-tight check.
+  # TODO: make rollup or another bundler write a depfile.
+  - ps: |-
+      $ignore = "test_util.ts", "unit_tests.ts", "*_test.ts"
+      Get-ChildItem "js" -File -Force -Name                               |
+        where   { $name = $_; -not ($ignore | where { $name -like $_ }) } |
+        where   { -not (Select-String -Pattern $_ -Path BUILD.gn `
+                                      -SimpleMatch -CaseSensitive) }      |
+        foreach { throw "$_ should be listed in BUILD.gn but isn't." }
+
   # Verify that generated ninja files do not use absolute path names.
   # If they do, it makes ccache/sccache much less effective.
   - ps: |-