Improve dartdoc for extent callbacks (#3514)

Fixes #3510

packages/flutter/lib/src/widgets/lazy_block.dart

@@ -201,6 +201,20 @@ class _LazyBlockState extends ScrollableState<LazyBlock> {
 }
 
 /// Signature used by [LazyBlockViewport] to report its interior and exterior dimensions.
+///
+///  * The [contentExtent] is the interior dimension of the viewport (i.e., the
+///    size of the thing that's being viewed through the viewport).
+///  * The [containerExtent] is the exterior dimension of the viewport (i.e.,
+///    the amount of the thing inside the viewport that is visible from outside
+///    the viewport).
+///  * The [minScrollOffset] is the offset at which the starting edge of the
+///    first item in the viewport is aligned with the starting edge of the
+///    viewport. (As the scroll offset increases, items with larger indices are
+///    revealed in the viewport.) Typically the min scroll offset is 0.0, but
+///    because [LazyBlockViewport] uses dead reckoning, the min scroll offset
+///    might not always be 0.0. For example, if an item that's offscreen changes
+///    size, the visible items will retain their current scroll offsets even if
+///    the distance to the starting edge of the first item changes.
 typedef void LazyBlockExtentsChangedCallback(double contentExtent, double containerExtent, double minScrollOffset);
 
 /// A viewport on an infinite list of variable height children.

packages/flutter/lib/src/widgets/virtual_viewport.dart

@@ -10,6 +10,13 @@ import 'framework.dart';
 
 import 'package:flutter/rendering.dart';
 
+/// Signature for reporting the interior and exterior dimensions of a viewport.
+///
+///  * The [contentExtent] is the interior dimension of the viewport (i.e., the
+///    size of the thing that's being viewed through the viewport).
+///  * The [containerExtent] is the exterior dimension of the viewport (i.e.,
+///    the amount of the thing inside the viewport that is visible from outside
+///    the viewport).
 typedef void ExtentsChangedCallback(double contentExtent, double containerExtent);
 
 /// An abstract widget whose children are not all materialized.

packages/flutter_sprites/lib/src/nine_slice_sprite.dart

@@ -81,7 +81,7 @@ class NineSliceSprite extends NodeWithSize with SpritePaint {
   List<Point> _vertices;
   List<Point> _textureCoordinates;
   List<Color> _colors;
-  List<int> _indicies;
+  List<int> _indices;
 
   @override
   void paint(Canvas canvas) {
@@ -157,7 +157,7 @@ class NineSliceSprite extends NodeWithSize with SpritePaint {
       }
 
       // Build indices.
-      _indicies = <int>[];
+      _indices = <int>[];
       for (int y = 0; y < 3; y += 1) {
         for (int x = 0; x < 3; x += 1) {
           // Check if we should skip the middle rectangle.
@@ -167,13 +167,13 @@ class NineSliceSprite extends NodeWithSize with SpritePaint {
           // Add a rectangle (two triangles).
           int index = y * 4 + x;
 
-          _indicies.add(index);
-          _indicies.add(index + 1);
-          _indicies.add(index + 4);
+          _indices.add(index);
+          _indices.add(index + 1);
+          _indices.add(index + 4);
 
-          _indicies.add(index + 1);
-          _indicies.add(index + 5);
-          _indicies.add(index + 4);
+          _indices.add(index + 1);
+          _indices.add(index + 5);
+          _indices.add(index + 4);
         }
       }
     }
@@ -184,7 +184,7 @@ class NineSliceSprite extends NodeWithSize with SpritePaint {
       _textureCoordinates,
       _colors,
       TransferMode.modulate,
-      _indicies,
+      _indices,
       _cachedPaint
     );
   }