Allow deprecated members from the Dart SDK and Flutter Engine to roll…

… in (#143347)

Namely, without breaking the tree. This is a deliberate policy decision
change.

See flutter/flutter#143312.

analysis_options.yaml

@@ -20,8 +20,10 @@ analyzer:
     strict-inference: true
     strict-raw-types: true
   errors:
-    # allow self-reference to deprecated members (we do this because otherwise we have
-    # to annotate every member in every test, assert, etc, when we deprecate something)
+    # allow deprecated members (we do this because otherwise we have to annotate
+    # every member in every test, assert, etc, when we or the Dart SDK deprecates
+    # something (https://github.com/flutter/flutter/issues/143312)
+    deprecated_member_use: ignore
     deprecated_member_use_from_same_package: ignore
   exclude:
     - "bin/cache/**"