More docs and other tweaks (#7026)

This fixes minor issues I ran into while using Flutter.

packages/flutter/lib/material.dart

@@ -28,6 +28,7 @@ export 'src/material/constants.dart';
 export 'src/material/data_table.dart';
 export 'src/material/data_table_source.dart';
 export 'src/material/date_picker.dart';
+export 'src/material/debug.dart';
 export 'src/material/dialog.dart';
 export 'src/material/divider.dart';
 export 'src/material/drawer.dart';

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

@@ -21,9 +21,9 @@ import 'theme.dart';
 ///
 /// See also:
 ///
-///  * [AlertDialog]
-///  * [SimpleDialog]
-///  * [showDialog]
+///  * [AlertDialog], for dialogs that have a message and some buttons.
+///  * [SimpleDialog], for dialogs that offer a variety of options.
+///  * [showDialog], which actually displays the dialog and returns its result.
 ///  * <https://material.google.com/components/dialogs.html>
 class Dialog extends StatelessWidget {
   /// Creates a dialog.
@@ -86,9 +86,9 @@ class Dialog extends StatelessWidget {
 ///
 /// See also:
 ///
-///  * [SimpleDialog]
-///  * [Dialog]
-///  * [showDialog]
+///  * [SimpleDialog], which handles the scrolling of the contents but has no [actions].
+///  * [Dialog], on which [AlertDialog] and [SimpleDialog] are based.
+///  * [showDialog], which actually displays the dialog and returns its result.
 ///  * <https://material.google.com/components/dialogs.html#dialogs-alerts>
 class AlertDialog extends StatelessWidget {
   /// Creates an alert dialog.
@@ -196,9 +196,9 @@ class AlertDialog extends StatelessWidget {
 ///
 /// See also:
 ///
-///  * [AlertDialog]
-///  * [Dialog]
-///  * [showDialog]
+///  * [AlertDialog], for dialogs that have a row of buttons below the body.
+///  * [Dialog], on which [SimpleDialog] and [AlertDialog] are based.
+///  * [showDialog], which actually displays the dialog and returns its result.
 ///  * <https://material.google.com/components/dialogs.html#dialogs-simple-dialogs>
 class SimpleDialog extends StatelessWidget {
   /// Creates a simple dialog.
@@ -320,7 +320,9 @@ class _DialogRoute<T> extends PopupRoute<T> {
 /// [Navigator.pop] when the dialog was closed.
 ///
 /// See also:
-///  * [Dialog]
+///  * [SimpleDialog], which handles the scrolling of the contents but has no [actions].
+///  * [AlertDialog], for dialogs that have a row of buttons below the body.
+///  * [Dialog], on which [SimpleDialog] and [AlertDialog] are based.
 ///  * <https://material.google.com/components/dialogs.html>
 Future<dynamic/*=T*/> showDialog/*<T>*/({ BuildContext context, Widget child }) {
   return Navigator.push(context, new _DialogRoute<dynamic/*=T*/>(

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

@@ -333,7 +333,7 @@ class _CircularProgressIndicatorState extends State<CircularProgressIndicator> w
     return new Container(
       constraints: new BoxConstraints(
         minWidth: _kMinCircularProgressIndicatorSize,
-        minHeight: _kMinCircularProgressIndicatorSize
+        minHeight: _kMinCircularProgressIndicatorSize,
       ),
       child: new CustomPaint(
         painter: new _CircularProgressIndicatorPainter(

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

@@ -11,20 +11,24 @@ import 'framework.dart';
 import 'ticker_provider.dart';
 import 'transitions.dart';
 
-/// Specifies which of the children to show. See [AnimatedCrossFade].
+/// Specifies which of two children to show. See [AnimatedCrossFade].
 ///
 /// The child that is shown will fade in, while the other will fade out.
 enum CrossFadeState {
-  /// Show the first child and hide the second.
+  /// Show the first child ([AnimatedCrossFade.firstChild]) and hide the second
+  /// ([AnimatedCrossFade.secondChild]]).
   showFirst,
-  /// Show the second child and hide the first.
-  showSecond
+  /// Show the second child ([AnimatedCrossFade.secondChild]) and hide the first
+  /// ([AnimatedCrossFade.firstChild]).
+  showSecond,
 }
 
-/// A widget that cross-fades between two children and animates itself between
-/// their sizes. The animation is controlled through the [crossFadeState]
-/// parameter. [firstCurve] and [secondCurve] represent the opacity curves of
-/// the two children. Note that [firstCurve] is inverted, i.e. it fades out when
+/// A widget that cross-fades between two given children and animates itself
+/// between their sizes.
+///
+/// The animation is controlled through the [crossFadeState] parameter.
+/// [firstCurve] and [secondCurve] represent the opacity curves of the two
+/// children. Note that [firstCurve] is inverted, i.e. it fades out when
 /// providing a growing curve like [Curves.linear]. [sizeCurve] is the curve
 /// used to animated between the size of the fading out child and the size of
 /// the fading in child.
@@ -82,8 +86,6 @@ class AnimatedCrossFade extends StatefulWidget {
 }
 
 class _AnimatedCrossFadeState extends State<AnimatedCrossFade> with TickerProviderStateMixin {
-  _AnimatedCrossFadeState() : super();
-
   AnimationController _controller;
   Animation<double> _firstAnimation;
   Animation<double> _secondAnimation;