Ignore some deprecated analyzer members (dart-lang/source_gen#726)

Unblock CI in the short term.

Author: natebosch

source_gen/source_gen/source_gen/lib/src/library.dart

@@ -37,8 +37,11 @@ class LibraryReader {
   Iterable<Element> get allElements => [
         element,
         ...element.topLevelElements,
+        // ignore: deprecated_member_use
         ...element.libraryImports,
+        // ignore: deprecated_member_use
         ...element.libraryExports,
+        // ignore: deprecated_member_use
         ...element.parts,
       ];