Flutter external 将方法的声明和实现分离

Flutter external 将方法的声明和实现分离, 查看 Map的源码如下 :

abstract class Map<K, V> {
  /// Creates an empty [LinkedHashMap].
  ///
  /// This constructor is equivalent to the non-const map literal `<K,V>{}`.
  ///
  /// A `LinkedHashMap` requires the keys to implement compatible
  /// `operator==` and `hashCode`.
  /// It iterates in key insertion order.
  external factory Map();

  /// Creates a [LinkedHashMap] with the same keys and values as [other].
  ///
  /// The keys must all be instances of [K] and the values of [V].
  /// The [other] map itself can have any type, unlike for [Map.of],
  /// and the key and value types are checked (and can fail) at run-time.
  ///
  /// Prefer using [Map.of] when possible, and only use `Map.from`
  /// to create a new map with more precise types than the original,
  /// and when it's known that all the keys and values have those
  /// more precise types.
  ///
  /// A `LinkedHashMap` requires the keys to implement compatible
  /// `operator==` and `hashCode`.
  /// It iterates in key insertion order.
  /// ```dart
  /// final planets = <num, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth', 4: 'Mars'};
  /// final mapFrom = Map<int, String>.from(planets);
  /// print(mapFrom); // {1: Mercury, 2: Venus, 3: Earth, 4: Mars}
  /// ```
  factory Map.from(Map other) = LinkedHashMap<K, V>.from;

  /// Creates a [LinkedHashMap] with the same keys and values as [other].
  ///
  /// A `LinkedHashMap` requires the keys to implement compatible
  /// `operator==` and `hashCode`, and it allows `null` as a key.
  /// It iterates in key insertion order.
  /// ```dart
  /// final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
  /// final mapOf = Map<num, String>.of(planets);
  /// print(mapOf); // {1: Mercury, 2: Venus, 3: Earth}
  /// ```
  factory Map.of(Map<K, V> other) = LinkedHashMap<K, V>.of;

  /// Creates an unmodifiable hash-based map containing the entries of [other].
  ///
  /// The keys must all be instances of [K] and the values of [V].
  /// The [other] map itself can have any type.
  ///
  /// The map requires the keys to implement compatible
  /// `operator==` and `hashCode`.
  /// The created map iterates keys in a fixed order,
  /// preserving the order provided by [other].
  ///
  /// The resulting map behaves like the result of [Map.from],
  /// except that the map returned by this constructor is not modifiable.
  /// ```dart
  /// final planets = <int, String>{1: 'Mercury', 2: 'Venus', 3: 'Earth'};
  /// final unmodifiableMap = Map.unmodifiable(planets);
  /// unmodifiableMap[4] = 'Mars'; // Throws
  /// ```
  external factory Map.unmodifiable(Map<dynamic, dynamic> other);

Map 的具体实现在 patch 方法中 , 之所以要分离声明和方法体, 可以在跨平台时实现不同的实现