Where is the definition of `map.clear()`?

My answer is based on the assumption that you want the implementation used when Dart is running natively and not on the web.

The default Map in Dart is a LinkedHashMap. There are several layers before getting the implementation of clear() but I expect this is the one you are looking for:

  void clear() {
    if (!isEmpty) {
      _index = _uninitializedIndex;
      _hashMask = _HashBase._UNINITIALIZED_HASH_MASK;
      _data = _uninitializedData;
      _usedData = 0;
      _deletedKeys = 0;
    }
  }

https://github.com/dart-lang/sdk/blob/2.15.1/sdk/lib/_internal/vm/lib/compact_hash.dart#L333-L341