Document various 'alignment' members so it's clear how to concretely specify it. (#13577)

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

@@ -108,6 +108,13 @@ class IconButton extends StatelessWidget {
   /// Defines how the icon is positioned within the IconButton.
   ///
   /// This property must not be null. It defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The icon to display inside the button.

packages/flutter/lib/src/painting/decoration_image.dart

@@ -86,6 +86,13 @@ class DecorationImage {
   /// when the image is painted.
   ///
   /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The center slice for a nine-patch image.

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

@@ -166,6 +166,13 @@ class AnimatedCrossFade extends StatefulWidget {
   /// How the children should be aligned while the size is animating.
   ///
   /// Defaults to [Alignment.topCenter].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// A builder that positions the [firstChild] and [secondChild] widgets.

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

@@ -34,6 +34,15 @@ class AnimatedSize extends SingleChildRenderObjectWidget {
   /// edge of the parent. Other values interpolate (and extrapolate) linearly.
   /// For example, a value of 0.0 means that the center of the child is aligned
   /// with the center of the parent.
+  ///
+  /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The animation curve when transitioning this widget's size to match the

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

@@ -1001,6 +1001,15 @@ class FittedBox extends SingleChildRenderObjectWidget {
   /// An alignment of (-1.0, -1.0) aligns the child to the top-left corner of its
   /// parent's bounds.  An alignment of (1.0, 0.0) aligns the child to the middle
   /// of the right edge of its parent's bounds.
+  ///
+  /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   @override
@@ -1672,7 +1681,7 @@ class UnconstrainedBox extends SingleChildRenderObjectWidget {
   /// be retained, and if set to [Axis.horizontal], then horizontal constraints
   /// will be retained.
   final Axis constrainedAxis;
-  
+
   @override
   void updateRenderObject(BuildContext context, covariant RenderUnconstrainedBox renderObject) {
     renderObject
@@ -1752,6 +1761,15 @@ class FractionallySizedBox extends SingleChildRenderObjectWidget {
   /// edge of the parent. Other values interpolate (and extrapolate) linearly.
   /// For example, a value of 0.0 means that the center of the child is aligned
   /// with the center of the parent.
+  ///
+  /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   @override
@@ -1886,6 +1904,15 @@ class OverflowBox extends SingleChildRenderObjectWidget {
   /// edge of the parent. Other values interpolate (and extrapolate) linearly.
   /// For example, a value of 0.0 means that the center of the child is aligned
   /// with the center of the parent.
+  ///
+  /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The minimum width constraint to give the child. Set this to null (the
@@ -1973,6 +2000,15 @@ class SizedOverflowBox extends SingleChildRenderObjectWidget {
   /// edge of the parent. Other values interpolate (and extrapolate) linearly.
   /// For example, a value of 0.0 means that the center of the child is aligned
   /// with the center of the parent.
+  ///
+  /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The size this widget should attempt to be.
@@ -2498,6 +2534,15 @@ class Stack extends MultiChildRenderObjectWidget {
   /// particular axis (e.g. that have neither `top` nor `bottom` set), use the
   /// alignment to determine how they should be positioned in that
   /// under-specified axis.
+  ///
+  /// Defaults to [AlignmentDirectional.topStart].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// The text direction with which to resolve [alignment].
@@ -4244,6 +4289,13 @@ class RawImage extends LeafRenderObjectWidget {
   /// must be in scope.
   ///
   /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// How to paint any portions of the layout bounds not covered by the image.

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

@@ -279,6 +279,13 @@ class Container extends StatelessWidget {
   /// constraints are unbounded, then the child will be shrink-wrapped instead.
   ///
   /// Ignored if [child] is null.
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// Empty space to inscribe inside the [decoration]. The [child], if any, is

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

@@ -251,6 +251,13 @@ class FadeInImage extends StatefulWidget {
   /// must be in scope.
   ///
   /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// How to paint any portions of the layout bounds not covered by the image.

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

@@ -391,6 +391,13 @@ class Image extends StatefulWidget {
   /// must be in scope.
   ///
   /// Defaults to [Alignment.center].
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// How to paint any portions of the layout bounds not covered by the image.

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

@@ -398,6 +398,13 @@ class AnimatedContainer extends ImplicitlyAnimatedWidget {
   /// constraints are unbounded, then the child will be shrink-wrapped instead.
   ///
   /// Ignored if [child] is null.
+  ///
+  /// See also:
+  ///
+  ///  * [Alignment], a class with convenient constants typically used to
+  ///    specify an [AlignmentGeometry].
+  ///  * [AlignmentDirectional], like [Alignment] for specifying alignments
+  ///    relative to text direction.
   final AlignmentGeometry alignment;
 
   /// Empty space to inscribe inside the [decoration]. The [child], if any, is
@@ -1144,7 +1151,7 @@ class _AnimatedPhysicalModelState extends AnimatedWidgetBaseState<AnimatedPhysic
       borderRadius: _borderRadius.evaluate(animation),
       elevation: _elevation.evaluate(animation),
       color: widget.animateColor ? _color.evaluate(animation) : widget.color,
-      shadowColor: widget.animateShadowColor 
+      shadowColor: widget.animateShadowColor
           ? _shadowColor.evaluate(animation)
           : widget.shadowColor,
     );