Fix test breakage in checked mode

tests/language/string_interpolation_and_buffer_test.dart

Index: tests/language/string_interpolation_and_buffer_test.dart
diff --git a/tests/language/string_interpolation_and_buffer_test.dart b/tests/language/string_interpolation_and_buffer_test.dart
index a0d6e12604fd407a81e4bb7ae699b4d6a790e1d4..fd2ad093fd7a7657d1380824cb678e98dfb0a648 100644
--- a/tests/language/string_interpolation_and_buffer_test.dart
+++ b/tests/language/string_interpolation_and_buffer_test.dart
@@ -27,6 +27,8 @@ main() {
         result = '${wrap(object)}';
       } on TypeError {
         return 'Error';
+      } on ArgumentError {
+        return 'Error';  // Checked mode.
       }
     } else {
       try {
@@ -46,6 +48,8 @@ main() {
         sb = new StringBuffer()..write(wrap(object));
       } on TypeError {
         return 'Error';
+      } on ArgumentError {
+        return 'Error';  // Checked mode.
       }
     } else {
       try {
@@ -65,6 +69,8 @@ main() {
         sb = new StringBuffer(wrap(object));
       } on TypeError {
         return 'Error';
+      } on ArgumentError {
+        return 'Error';  // Checked mode.
       }
     } else {
       try {