Hides libraries that should not be documented.

pkg/docgen/lib/docgen.dart

Index: pkg/docgen/lib/docgen.dart
diff --git a/pkg/docgen/lib/docgen.dart b/pkg/docgen/lib/docgen.dart
index b2568dff81b442f8789ac0500330b753377c6037..bca15b6f0def7788d96289be2171d5bb9d8442c0 100644
--- a/pkg/docgen/lib/docgen.dart
+++ b/pkg/docgen/lib/docgen.dart
@@ -277,7 +277,11 @@ void _writeIndexableToFile(Indexable result, bool outputToYaml) {
  */
 // This is because LibraryMirror.isPrivate returns `false` all the time.
 bool _isLibraryPrivate(LibraryMirror mirror) {
-  if (mirror.simpleName.startsWith('_') || mirror.simpleName.contains('._')) {
+  var sdkLibrary = LIBRARIES[mirror.simpleName];
+  if (sdkLibrary != null) {
+    return !sdkLibrary.documented;
+  } else if (mirror.simpleName.startsWith('_') || 
+      mirror.simpleName.contains('._')) {
     return true;
   }
   return false;