Remove deprecated Strings class.

tests/utils/test_utils.dart

Index: tests/utils/test_utils.dart
diff --git a/tests/utils/test_utils.dart b/tests/utils/test_utils.dart
index 40a2e5502f7cd4c62b27bba30d7cbeb4f933f2d2..a786fa2846365d2168670d3e89ecb8e381e576cb 100644
--- a/tests/utils/test_utils.dart
+++ b/tests/utils/test_utils.dart
@@ -30,7 +30,7 @@ String cleanUpLiteral(String text) {
     }
   }
 
-  return Strings.join(lines, '\n');
+  return lines.join('\n');
 }
 
 /**
@@ -49,5 +49,5 @@ String indentLiteral(String text) {
     lines[i] = "        ${lines[i]}";
   }
 
-  return Strings.join(lines, "\n");
+  return lines.join("\n");
 }