Fix analysis_test "filter completions" test

[biggs0125]

This is producing the following on the main branch:

test/presubmit/analysis_test.dart: analysis filter completions (failed)
  [Analyzer RequestError method: completion.getSuggestions2, code: SERVER_ERROR, message: Bad state: No element]

[devoncarew]

For this test, we're verifying that we get some results back from code completion. We're completing in:

class A {}
class AB {}
class ABC {}
void main() {
  var c = A
}
class ZZ {}
class a {}

near the end of the var c = A line, right before the A.

We expect to get a number of results back, including A, AB, ABC, ... but are seeing no results. Note that the dev and stable channels still report results, just not main.

cc @keertip @bwilkerson @scheglov