[flutter_tools] remove build script from tool package (#50690)

packages/_flutter_web_build_script/analysis_options.yaml

diff --git a/packages/_flutter_web_build_script/analysis_options.yaml b/packages/_flutter_web_build_script/analysis_options.yaml
new file mode 100644
index 0000000..525929f
--- /dev/null
+++ b/packages/_flutter_web_build_script/analysis_options.yaml
@@ -0,0 +1,9 @@
+# Use the analysis options settings from the top level of the repo (not
+# the ones from above, which include the `public_member_api_docs` rule).
+
+include: ../../analysis_options.yaml
+
+linter:
+  rules:
+    unawaited_futures: true
+    curly_braces_in_flow_control_structures: true