--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -689,20 +689,25 @@ class TreeMetadataEmitter(LoggingMixin):
                     yield cls(*arglist)
 
         for f, flags in all_flags.iteritems():
             if flags.flags:
                 ext = mozpath.splitext(f)[1]
                 yield PerSourceFlag(context, f, flags.flags)
 
 
-    def emit_from_context(self, context):
+    def emit_from_context(self, context, reading_test_manifests=False):
         """Convert a Context to tree metadata objects.
 
         This is a generator of mozbuild.frontend.data.ContextDerived instances.
+
+        If ``reading_test_manifests`` is True, we short circuit after test
+        manifests are read. It is meant to be used as a hack for moz.build
+        file reading mode, which gets test metadata from emitter because the
+        abstraction is wrong.
         """
 
         # We only want to emit an InstallationTarget if one of the consulted
         # variables is defined. Later on, we look up FINAL_TARGET, which has
         # the side-effect of populating it. So, we need to do this lookup
         # early.
         if any(k in context for k in ('FINAL_TARGET', 'XPI_NAME', 'DIST_SUBDIR')):
             yield InstallationTarget(context)
@@ -899,16 +904,20 @@ class TreeMetadataEmitter(LoggingMixin):
                                                    mozpath.basename(c)))
 
         for obj in self._handle_linkables(context, passthru):
             yield obj
 
         for obj in self._process_test_manifests(context):
             yield obj
 
+        # Early return when all we care about is test manifests.
+        if reading_test_manifests:
+            return
+
         for obj in self._process_jar_manifests(context):
             yield obj
 
         for name, jar in context.get('JAVA_JAR_TARGETS', {}).items():
             yield ContextWrapped(context, jar)
 
         for name, data in context.get('ANDROID_ECLIPSE_PROJECT_TARGETS', {}).items():
             yield ContextWrapped(context, data)

--- a/python/mozbuild/mozbuild/frontend/reader.py
+++ b/python/mozbuild/mozbuild/frontend/reader.py
@@ -1414,17 +1414,18 @@ class TestContextReader(object):
         for ctx in ctxs:
             test_context = Context(VARIABLES, self.config)
             test_context.main_path = ctx.main_path
 
             # Clone just the keys that will result in test manifests.
             manifest_keys = [key for key in ctx if key in self._test_manifest_contexts]
             for key in manifest_keys:
                 test_context[key] = ctx[key]
-            for obj in emitter.emit_from_context(test_context):
+            for obj in emitter.emit_from_context(test_context,
+                                                 reading_test_manifests=True):
                 if isinstance(obj, TestManifest):
                     for t in obj.tests:
                         if 'relpath' not in t:
                             # wpt manifests do not generate relpaths (bug 1207678).
                             t['relpath'] = mozpath.relpath(t['path'],
                                                            self.config.topsrcdir)
                         # Pull in the entire directory of tests.
                         result_context.test_files.add(mozpath.dirname(t['relpath']) + '/**')