Aspects that apply to files cannot apply to source files.

Make them not crash.

RELNOTES: None.
PiperOrigin-RevId: 164265379
diff --git a/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java b/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
index 4b8750d..7fcd60f 100644
--- a/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
+++ b/src/main/java/com/google/devtools/build/lib/packages/AspectDefinition.java
@@ -159,9 +159,10 @@
   }
 
   /**
-   * Returns whether this aspect applies to files.
+   * Returns whether this aspect applies to (output) files.
    *
-   * Currently only supported for top-level aspects and targets.
+   * Currently only supported for top-level aspects and targets, and
+   * only for output files.
    */
   public boolean applyToFiles() {
     return applyToFiles;
@@ -459,7 +460,8 @@
      * Sets whether this aspect should apply to files.
      *
      * Default is <code>false</code>.
-     * Currently only supported for top-level aspects and targets.
+     * Currently only supported for top-level aspects and targets, and only for
+     * output files.
      */
     public Builder applyToFiles(boolean propagateOverGeneratedFiles) {
       this.applyToFiles = propagateOverGeneratedFiles;