Declare the package strong-mode clean.

R=jmesserly@google.com

Review URL: https://codereview.chromium.org//1955433002 .

pkgs/source_span/.analysis_options

diff --git a/pkgs/source_span/.analysis_options b/pkgs/source_span/.analysis_options
new file mode 100644
index 0000000..a10d4c5
--- /dev/null
+++ b/pkgs/source_span/.analysis_options
@@ -0,0 +1,2 @@
+analyzer:
+  strong-mode: true

pkgs/source_span/test/utils_test.dart

diff --git a/pkgs/source_span/test/utils_test.dart b/pkgs/source_span/test/utils_test.dart
index 5d973f5..2a86cc0 100644
--- a/pkgs/source_span/test/utils_test.dart
+++ b/pkgs/source_span/test/utils_test.dart
@@ -45,11 +45,3 @@
     });
   });
 }
-
-_linearSearch(list, predicate) {
-  if (list.length == 0) return -1;
-  for (int i = 0; i < list.length; i++) {
-    if (predicate(list[i])) return i;
-  }
-  return list.length;
-}