[analyzer] Missing/incorrect completion for analysis_options (in some cases)

[incendial]

Hi, this is probably related to yaml parser, but still.

1. When invoking the completion suggestions manually (in my case, option + esc), top level entries are suggested even when they are already in the config:

To reproduce:

analyzer:
  // <-- call the completion here
  exclude:

2. Typing anything under the first entry (when there are other entries below it) does not trigger the completion at all

analyzer:
  // <-- start typing here
  exclude:

Expected suggestions:

[incendial]

@DanTup fyi

[dart-github-bot]

Summary: The Dart analyzer's code completion for analysis_options.yaml is inconsistent. It suggests top-level entries even when they exist, and fails to trigger completion when typing under existing entries.