Some cleanup of Chips that I found while learning the code. (#16249)

packages/flutter/lib/src/material/chip.dart

@@ -64,6 +64,10 @@ const Icon _kDefaultDeleteIcon = const Icon(Icons.cancel, size: _kDeleteIconSize
 ///  * [ActionChip], represents an action related to primary content.
 ///  * <https://material.google.com/components/chips.html>
 abstract class ChipAttributes {
+  // This class is intended to be used as an interface, and should not be
+  // extended directly.
+  factory ChipAttributes._() => null;
+
   /// The primary content of the chip.
   ///
   /// Typically a [Text] widget.
@@ -121,6 +125,10 @@ abstract class ChipAttributes {
 ///    compact form.
 ///  * <https://material.google.com/components/chips.html>
 abstract class DeletableChipAttributes {
+  // This class is intended to be used as an interface, and should not be
+  // extended directly.
+  factory DeletableChipAttributes._() => null;
+
   /// The icon displayed when [onDeleted] is set.
   ///
   /// Defaults to an [Icon] widget set to use [Icons.cancel].
@@ -212,6 +220,10 @@ abstract class DeletableChipAttributes {
 ///  * [FilterChip], uses tags or descriptive words as a way to filter content.
 ///  * <https://material.google.com/components/chips.html>
 abstract class SelectableChipAttributes {
+  // This class is intended to be used as an interface, and should not be
+  // extended directly.
+  factory SelectableChipAttributes._() => null;
+
   /// Whether or not this chip is selected.
   ///
   /// If [onSelected] is not null, this value will be used to determine if the
@@ -290,6 +302,10 @@ abstract class SelectableChipAttributes {
 ///  * [FilterChip], uses tags or descriptive words as a way to filter content.
 ///  * <https://material.google.com/components/chips.html>
 abstract class DisabledChipAttributes {
+  // This class is intended to be used as an interface, and should not be
+  // extended directly.
+  factory DisabledChipAttributes._() => null;
+
   /// Whether or not this chip is enabled for input.
   ///
   /// If this is true, but all of the user action callbacks are null (i.e.
@@ -333,6 +349,10 @@ abstract class DisabledChipAttributes {
 ///  * [ActionChip], represents an action related to primary content.
 ///  * <https://material.google.com/components/chips.html>
 abstract class TappableChipAttributes {
+  // This class is intended to be used as an interface, and should not be
+  // extended directly.
+  factory TappableChipAttributes._() => null;
+
   /// Called when the user taps the chip.
   ///
   /// If [onPressed] is set, then this callback will be called when the user
@@ -372,7 +392,7 @@ abstract class TappableChipAttributes {
 /// button for deleting the chip.
 ///
 /// Requires one of its ancestors to be a [Material] widget. The [label],
-/// [deleteIcon] and [border] arguments must not be null.
+/// [deleteIcon], and [border] arguments must not be null.
 ///
 /// ## Sample code
 ///
@@ -402,7 +422,7 @@ abstract class TappableChipAttributes {
 class Chip extends StatelessWidget implements ChipAttributes, DeletableChipAttributes {
   /// Creates a material design chip.
   ///
-  /// The [label], [deleteIcon] and [border] arguments must not be null.
+  /// The [label], [deleteIcon], and [border] arguments must not be null.
   const Chip({
     Key key,
     this.avatar,
@@ -634,7 +654,7 @@ class InputChip extends StatelessWidget
 /// related descriptive text or categories.
 ///
 /// Requires one of its ancestors to be a [Material] widget. The [selected],
-/// [label] and [border] arguments must not be null.
+/// [label], and [border] arguments must not be null.
 ///
 /// ## Sample code
 ///
@@ -684,6 +704,9 @@ class ChoiceChip extends StatelessWidget
         ChipAttributes,
         SelectableChipAttributes,
         DisabledChipAttributes {
+  /// Create a chip that acts like a radio button.
+  ///
+  /// The [selected], [label], and [border] arguments must not be null.
   const ChoiceChip({
     Key key,
     this.avatar,
@@ -849,6 +872,9 @@ class FilterChip extends StatelessWidget
         ChipAttributes,
         SelectableChipAttributes,
         DisabledChipAttributes {
+  /// Create a chip that acts like a checkbox.
+  ///
+  /// The [selected], [label], and [border] arguments must not be null.
   const FilterChip({
     Key key,
     this.avatar,
@@ -927,17 +953,19 @@ class FilterChip extends StatelessWidget
 /// content. Action chips should appear dynamically and contextually in a UI.
 ///
 /// Action chips can be tapped to trigger an action or show progress and
-/// confirmation.
+/// confirmation. They cannot be disabled; if the action is not applicable, the
+/// chip should not be included in the interface. (This contrasts with buttons,
+/// where unavailable choices are usually represented as disabled controls.)
 ///
 /// Action chips are displayed after primary content, such as below a card or
 /// persistently at the bottom of a screen.
 ///
-/// The material button widgets, [RaisedButton], [FlatButton], [OutlineButton]
-/// are an alternative to action chips, which should appear statically and
-/// consistently in a UI.
+/// The material button widgets, [RaisedButton], [FlatButton], and
+/// [OutlineButton], are an alternative to action chips, which should appear
+/// statically and consistently in a UI.
 ///
 /// Requires one of its ancestors to be a [Material] widget. The [onPressed],
-/// [label] and [border] arguments must not be null.
+/// [label], and [border] arguments must not be null.
 ///
 /// ## Sample code
 ///
@@ -967,6 +995,9 @@ class FilterChip extends StatelessWidget
 ///    vertical runs.
 ///  * <https://material.google.com/components/chips.html>
 class ActionChip extends StatelessWidget implements ChipAttributes, TappableChipAttributes {
+  /// Create a chip that acts like a button.
+  ///
+  /// The [label], [border], and [onPressed] arguments must not be null.
   const ActionChip({
     Key key,
     this.avatar,
@@ -981,7 +1012,11 @@ class ActionChip extends StatelessWidget implements ChipAttributes, TappableChip
     this.padding,
   })  : assert(label != null),
         assert(shape != null),
-        assert(onPressed != null),
+        assert(
+          onPressed != null,
+          'Rather than disabling an ActionChip by setting onPressed to null, '
+          'remove it from the interface entirely.',
+        ),
         super(key: key);
 
   @override