--- a/dom/bindings/parser/WebIDL.py
+++ b/dom/bindings/parser/WebIDL.py
@@ -1064,16 +1064,19 @@ class IDLInterface(IDLObjectWithScope):
                     m.isMethod() and not m.isStatic())) == 1)
 
     def isExposedInWindow(self):
         return 'Window' in self.exposureSet
 
     def isExposedInAnyWorker(self):
         return len(self.getWorkerExposureSet()) > 0
 
+    def isExposedInSystemGlobals(self):
+        return 'BackstagePass' in self.exposureSet
+
     def isExposedOnlyInSomeWorkers(self):
         assert self.isExposedInAnyWorker()
         workerScopes = self.parentScope.globalNameMapping["Worker"]
         return len(workerScopes.difference(self.exposureSet)) > 0
 
     def getWorkerExposureSet(self):
         workerScopes = self.parentScope.globalNameMapping["Worker"]
         return workerScopes.intersection(self.exposureSet)
@@ -5604,16 +5607,20 @@ class Parser(Tokenizer):
                             )
         logger.reportGrammarErrors()
 
         self._globalScope = IDLScope(BuiltinLocation("<Global Scope>"), None, None)
         # To make our test harness work, pretend like we have a primary global already.  Note that we _don't_ set _globalScope.primaryGlobalAttr, so we'll still be able to detect multiple PrimaryGlobal extended attributes.
         self._globalScope.primaryGlobalName = "FakeTestPrimaryGlobal"
         self._globalScope.globalNames.add("FakeTestPrimaryGlobal")
         self._globalScope.globalNameMapping["FakeTestPrimaryGlobal"].add("FakeTestPrimaryGlobal")
+        # And we add the special-cased "System" global name, which
+        # doesn't have any corresponding interfaces.
+        self._globalScope.globalNames.add("System")
+        self._globalScope.globalNameMapping["System"].add("BackstagePass")
         self._installBuiltins(self._globalScope)
         self._productions = []
 
         self._filename = "<builtin>"
         self.lexer.input(Parser._builtins)
         self._filename = None
 
         self.parser.parse(lexer=self.lexer,tracking=True)