Support aspect() definitions in Stardoc.

This does nothing meaningful with the aspect() invocation, but results in no error being thrown if an aspect definition exists in a target bzl file.

RELNOTES: None.
PiperOrigin-RevId: 210544862

src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java

diff --git a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
index f86c455..a01d018 100644
--- a/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
+++ b/src/main/java/com/google/devtools/build/skydoc/fakebuildapi/FakeSkylarkRuleFunctionsApi.java
@@ -119,7 +119,7 @@
       Object attrs, SkylarkList<?> requiredAspectProvidersArg, SkylarkList<?> providesArg,
       SkylarkList<?> fragments, SkylarkList<?> hostFragments, SkylarkList<?> toolchains, String doc,
       FuncallExpression ast, Environment funcallEnv) throws EvalException {
-    return null;
+    return new FakeSkylarkAspect();
   }
 
   /**