[O] Cleanup (#23530)

* Improve documentation

* Potential performance win in hit testing

Rather than copying the list every time we access `path`, this just
exposes it as an `Iterable`. People who want to copy it can use
`toList()`.

(This isn't a breaking change since code that expects a `List` is
still going to get one.)

* Enforce the Oxford Comma.

packages/flutter/lib/src/foundation/debug.dart

@@ -63,8 +63,13 @@ Future<T> debugInstrumentAction<T>(String description, Future<T> action()) {
   }
 }
 
-/// Arguments to whitelist [Timeline] events in order to be shown in the
-/// developer centric version of the Observatory Timeline.
+/// Argument passed to [Timeline] events in order to cause those events to be
+/// shown in the developer-centric version of the Observatory Timeline.
+///
+/// See also:
+///
+///  * [Timeline.startSync], which typically takes this value as its `arguments`
+///    argument.
 const Map<String, String> timelineWhitelistArguments = <String, String>{
   'mode': 'basic'
 };

packages/flutter/lib/src/gestures/hit_test.dart

@@ -66,7 +66,7 @@ class HitTestResult {
   /// The first entry in the path is the most specific, typically the one at
   /// the leaf of tree being hit tested. Event propagation starts with the most
   /// specific (i.e., first) entry and proceeds in order through the path.
-  List<HitTestEntry> get path => List<HitTestEntry>.unmodifiable(_path);
+  Iterable<HitTestEntry> get path => _path;
   final List<HitTestEntry> _path;
 
   /// Add a [HitTestEntry] to the path.

packages/flutter/lib/src/gestures/scale.dart

@@ -140,10 +140,10 @@ class _LineBetweenPointers{
 /// Recognizes a scale gesture.
 ///
 /// [ScaleGestureRecognizer] tracks the pointers in contact with the screen and
-/// calculates their focal point, indicated scale and rotation. When a focal pointer is
-/// established, the recognizer calls [onStart]. As the focal point, scale, rotation
-/// change, the recognizer calls [onUpdate]. When the pointers are no longer in
-/// contact with the screen, the recognizer calls [onEnd].
+/// calculates their focal point, indicated scale, and rotation. When a focal
+/// pointer is established, the recognizer calls [onStart]. As the focal point,
+/// scale, rotation change, the recognizer calls [onUpdate]. When the pointers
+/// are no longer in contact with the screen, the recognizer calls [onEnd].
 class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   /// Create a gesture recognizer for interactions intended for scaling content.
   ScaleGestureRecognizer({ Object debugOwner }) : super(debugOwner: debugOwner);

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

@@ -14,8 +14,8 @@ import 'framework.dart';
 /// [Flex.mainAxisAlignment] on a flex container that contains a [Spacer] to
 /// [MainAxisAlignment.spaceAround], [MainAxisAlignment.spaceBetween], or
 /// [MainAxisAlignment.spaceEvenly] will not have any visible effect: the
-/// [Spacer] has taken up all of the additional space, so there is none left to
-/// redistribute.
+/// [Spacer] has taken up all of the additional space, therefore there is none
+/// left to redistribute.
 ///
 /// ## Sample code
 ///
@@ -59,10 +59,7 @@ class Spacer extends StatelessWidget {
   Widget build(BuildContext context) {
     return Expanded(
       flex: flex,
-      child: const SizedBox(
-        height: 0.0,
-        width: 0.0,
-      ),
+      child: const SizedBox.shrink(),
     );
   }
 }