--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -149,23 +149,24 @@ class TreeMetadataEmitter(LoggingMixin):
 
     def summary(self):
         return ExecutionSummary(
             'Processed into {object_count:d} build config descriptors in '
             '{execution_time:.2f}s',
             execution_time=self._emitter_time,
             object_count=self._object_count)
 
-    def emit(self, output):
+    def emit(self, output, emitfn=None):
         """Convert the BuildReader output into data structures.
 
         The return value from BuildReader.read_topsrcdir() (a generator) is
         typically fed into this function.
         """
         contexts = {}
+        emitfn = emitfn or self.emit_from_context
 
         def emit_objs(objs):
             for o in objs:
                 self._object_count += 1
                 yield o
 
         for out in output:
             # Nothing in sub-contexts is currently of interest to us. Filter
@@ -174,17 +175,17 @@ class TreeMetadataEmitter(LoggingMixin):
                 continue
 
             if isinstance(out, Context):
                 # Keep all contexts around, we will need them later.
                 contexts[out.objdir] = out
 
                 start = time.time()
                 # We need to expand the generator for the timings to work.
-                objs = list(self.emit_from_context(out))
+                objs = list(emitfn(out))
                 self._emitter_time += time.time() - start
 
                 for o in emit_objs(objs): yield o
 
             else:
                 raise Exception('Unhandled output type: %s' % type(out))
 
         # Don't emit Linkable objects when COMPILE_ENVIRONMENT is not set