Fine. Tracking for LibraryElementImpl.publicNamespace

It turned out that it is de-facto not used, so we can get away with
recording opaque API usage.

Change-Id: I65bc1ec144b9cb280aaaf10b2b11bf96fc4a6e88
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/449982
Reviewed-by: Johnni Winther <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/element/element.dart

@@ -5888,6 +5888,7 @@ class LabelFragmentImpl extends FragmentImpl implements LabelFragment {
 }
 
 /// A concrete implementation of [LibraryElement].
+@elementClass
 class LibraryElementImpl extends ElementImpl
     with DeferredResolutionReadingMixin
     implements LibraryElement {
@@ -6372,7 +6373,9 @@ class LibraryElementImpl extends ElementImpl
   LibraryElementImpl get nonSynthetic => this;
 
   @override
+  @trackedDirectlyOpaque
   Namespace get publicNamespace {
+    globalResultRequirements?.recordOpaqueApiUse(this, 'publicNamespace');
     return _publicNamespace ??= NamespaceBuilder()
         .createPublicNamespaceForLibrary(this);
   }