Add an analysis_options.yaml file for test_runner.

I didn't realize they were being gitignored across the entire SDK repo.
I'm not sure why they are ignored at all, but based on the linked bug,
I think it's only a top-level analysis_options.yaml file that would
cause problems?

This change to the .gitignore file prohibits that case while still
allowing analysis_options.yaml files in subdirectories.

I very strongly want to have an options file for test_runner to ensure
that everyone on the team keeps it "no-implicit-cast" clean.

Change-Id: I9ceac507b08a063e71e2fe39501161723651e6b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/106840
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

.gitignore

@@ -46,16 +46,15 @@ cmake-build-debug
 .vscode
 .history
 
+# https://github.com/Dart-Code/Dart-Code/issues/1295
+/analysis_options.yaml
+
 # analysis server files
-analysis_options.yaml
 compile_commands.json
 
 # GDB files
 .gdb_history
 
-# https://github.com/Dart-Code/Dart-Code/issues/1295
-analysis_options.yaml
-
 # Built by chromebot and downloaded from Google Storage
 client/tests/drt
 

pkg/test_runner/analysis_options.yaml

@@ -0,0 +1,3 @@
+analyzer:
+  strong-mode:
+    implicit-casts: false