implement Iterator and Comparable instead of extending them (flutter#123282)

implement Iterator and Comparable instead of extending them

Author: jakemac53

dev/integration_tests/flutter_gallery/lib/demo/transformations/transformations_demo_board.dart

@@ -200,7 +200,7 @@ class Board extends IterableMixin<BoardPoint?> {
   }
 }
 
-class _BoardIterator extends Iterator<BoardPoint?> {
+class _BoardIterator implements Iterator<BoardPoint?> {
   _BoardIterator(this.boardPoints);
 
   final List<BoardPoint> boardPoints;

packages/flutter/lib/src/rendering/editable.dart

@@ -131,7 +131,7 @@ class TextSelectionPoint {
 /// the [VerticalCaretMovementRun] must not be used. The [isValid] property must
 /// be checked before calling [movePrevious], [moveNext] and [moveByOffset],
 /// or accessing [current].
-class VerticalCaretMovementRun extends Iterator<TextPosition> {
+class VerticalCaretMovementRun implements Iterator<TextPosition> {
   VerticalCaretMovementRun._(
     this._editable,
     this._lineMetrics,

packages/flutter/lib/src/semantics/semantics.dart

@@ -2875,7 +2875,7 @@ class _BoxEdge implements Comparable<_BoxEdge> {
 /// nodes that share the same [SemanticsNode] parent.
 ///
 /// The [nodes] are sorted among each other separately from other nodes.
-class _SemanticsSortGroup extends Comparable<_SemanticsSortGroup> {
+class _SemanticsSortGroup implements Comparable<_SemanticsSortGroup> {
   _SemanticsSortGroup({
     required this.startOffset,
     required this.textDirection,