Migrate `package:ffi`s `allocate` and `free` to `calloc`

We're deprecating `allocate` and `free` in https://github.com/dart-lang/ffi/tree/api/allocator-opaque.
diff --git a/test/number_format_compact_icu_test.dart b/test/number_format_compact_icu_test.dart
index 250b580..10bfba6 100644
--- a/test/number_format_compact_icu_test.dart
+++ b/test/number_format_compact_icu_test.dart
@@ -161,12 +161,12 @@
   // if (U_FAILURE(ec)) { return; }
   final cLocale = Utf8.toUtf8(locale);
   final cSkeleton = Utf16.toUtf16(skeleton);
-  final cErrorCode = allocate<Int32>(count: 1);
+  final cErrorCode = calloc<Int32>();
   cErrorCode.value = 0;
   final uformatter =
       unumf_openForSkeletonAndLocale!(cSkeleton, -1, cLocale, cErrorCode);
-  free(cSkeleton);
-  free(cLocale);
+  calloc.free(cSkeleton);
+  calloc.free(cLocale);
   var errorCode = cErrorCode.value;
   expect(errorCode, lessThanOrEqualTo(0),
       reason: u_errorName!(errorCode).toString());
@@ -201,7 +201,7 @@
   expect(errorCode, equals(15), // U_BUFFER_OVERFLOW_ERROR
       reason: u_errorName!(errorCode).toString());
   cErrorCode.value = 0;
-  final buffer = allocate<Utf16>(count: reqLen + 1);
+  final buffer = calloc<Utf16>(reqLen + 1);
   unumf_resultToString!(uresult, buffer, reqLen + 1, cErrorCode);
   errorCode = cErrorCode.value;
   expect(errorCode, lessThanOrEqualTo(0),
@@ -211,11 +211,11 @@
   // // Cleanup:
   // unumf_close(uformatter);
   // unumf_closeResult(uresult);
-  // free(buffer);
+  // calloc.free(buffer);
   unumf_close!(uformatter);
   unumf_closeResult!(uresult);
-  free(buffer);
-  free(cErrorCode);
+  calloc.free(buffer);
+  calloc.free(cErrorCode);
 
   return result;
 }