Automatically flip material icons in RTL. (#12876)

I'm not really sure how to test this without a reference test, since
eventually we want to move this to a font feature and thus the obvious
test (looking for the Transform widget) isn't really valid.

bin/internal/material_fonts.version

-https://storage.googleapis.com/flutter_infra/flutter/fonts/13ac995daa9dda0a6ba0a45f1fccc541e616a74c/fonts.zip
+https://storage.googleapis.com/flutter_infra/flutter/fonts/7a25ab00acc9a9e4a498641dcbd5a923a1249239/fonts.zip

dev/tools/update_icons.dart

@@ -28,6 +28,82 @@ const Map<String, String> kIdentifierRewrites = const <String, String>{
   'class': 'class_',
 };
 
+final Set<String> kMirroredIcons = new Set<String>.from(<String>[
+  // This list is obtained from:
+  // http://google.github.io/material-design-icons/#icons-in-rtl
+  'arrow_back',
+  'arrow_back_ios',
+  'arrow_forward',
+  'arrow_forward_ios',
+  'arrow_left',
+  'arrow_right',
+  'assignment',
+  'assignment_return',
+  'backspace',
+  'battery_unknown',
+  'call_made',
+  'call_merge',
+  'call_missed',
+  'call_missed_outgoing',
+  'call_received',
+  'call_split',
+  'chevron_left',
+  'chevron_right',
+  'chrome_reader_mode',
+  'device_unknown',
+  'dvr',
+  'event_note',
+  'featured_play_list',
+  'featured_video',
+  'first_page',
+  'flight_land',
+  'flight_takeoff',
+  'format_indent_decrease',
+  'format_indent_increase',
+  'format_list_bulleted',
+  'forward',
+  'functions',
+  'help',
+  'help_outline',
+  'input',
+  'keyboard_backspace',
+  'keyboard_tab',
+  'label',
+  'label_important',
+  'label_outline',
+  'last_page',
+  'launch',
+  'list',
+  'live_help',
+  'mobile_screen_share',
+  'multiline_chart',
+  'navigate_before',
+  'navigate_next',
+  'next_week',
+  'note',
+  'open_in_new',
+  'playlist_add',
+  'queue_music',
+  'redo',
+  'reply',
+  'reply_all',
+  'screen_share',
+  'send',
+  'short_text',
+  'show_chart',
+  'sort',
+  'star_half',
+  'subject',
+  'trending_flat',
+  'toc',
+  'trending_down',
+  'trending_up',
+  'undo',
+  'view_list',
+  'view_quilt',
+  'wrap_text',
+]);
+
 void main(List<String> args) {
   // If we're run from the `tools` dir, set the cwd to the repo root.
   if (path.basename(Directory.current.path) == 'tools')
@@ -94,9 +170,10 @@ String getIconDeclaration(String line) {
   final String codepoint = tokens[1];
   final String identifier = kIdentifierRewrites[name] ?? name;
   final String description = name.replaceAll('_', ' ');
+  final String rtl = kMirroredIcons.contains(name) ? ', matchTextDirection: true' : '';
   return '''
 
   /// <p><i class="material-icons md-36">$name</i> &#x2014; material icon named "$description".</p>
-  static const IconData $identifier = const IconData(0x$codepoint, fontFamily: 'MaterialIcons');
+  static const IconData $identifier = const IconData(0x$codepoint, fontFamily: 'MaterialIcons'$rtl);
 ''';
 }

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

@@ -599,31 +599,8 @@ class _MonthPickerState extends State<MonthPicker> {
     });
   }
 
-  Icon _getPreviousMonthIcon(TextDirection textDirection) {
-    assert(textDirection != null);
-    switch (textDirection) {
-      case TextDirection.rtl:
-        return const Icon(Icons.chevron_right);
-      case TextDirection.ltr:
-        return const Icon(Icons.chevron_left);
-    }
-    return null;
-  }
-
-  Icon _getNextMonthIcon(TextDirection textDirection) {
-    assert(textDirection != null);
-    switch (textDirection) {
-      case TextDirection.rtl:
-        return const Icon(Icons.chevron_left);
-      case TextDirection.ltr:
-        return const Icon(Icons.chevron_right);
-    }
-    return null;
-  }
-
   @override
   Widget build(BuildContext context) {
-    final TextDirection textDirection = Directionality.of(context);
     final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     return new SizedBox(
       width: _kMonthPickerPortraitWidth,
@@ -642,7 +619,7 @@ class _MonthPickerState extends State<MonthPicker> {
             top: 0.0,
             start: 8.0,
             child: new IconButton(
-              icon: _getPreviousMonthIcon(textDirection),
+              icon: const Icon(Icons.chevron_left),
               tooltip: localizations.previousMonthTooltip,
               onPressed: _isDisplayingFirstMonth ? null : _handlePreviousMonth,
             ),
@@ -651,7 +628,7 @@ class _MonthPickerState extends State<MonthPicker> {
             top: 0.0,
             end: 8.0,
             child: new IconButton(
-              icon: _getNextMonthIcon(textDirection),
+              icon: const Icon(Icons.chevron_right),
               tooltip: localizations.nextMonthTooltip,
               onPressed: _isDisplayingLastMonth ? null : _handleNextMonth,
             ),

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

@@ -11,7 +11,6 @@ import 'icon_data.dart';
 import 'icon_theme.dart';
 import 'icon_theme_data.dart';
 
-
 /// A graphical icon widget drawn with a glyph from a font described in
 /// an [IconData] such as material's predefined [IconData]s in [Icons].
 ///
@@ -37,6 +36,7 @@ class Icon extends StatelessWidget {
     this.size,
     this.color,
     this.semanticLabel,
+    this.textDirection,
   }) : super(key: key);
 
   /// The icon to display. The available icons are described in [Icons].
@@ -91,14 +91,29 @@ class Icon extends StatelessWidget {
   ///
   /// See also:
   ///
-  ///  * [Semantics.label] which is set with [semanticLabel] in the underlying
+  ///  * [Semantics.label], which is set to [semanticLabel] in the underlying
   ///    [Semantics] widget.
   final String semanticLabel;
 
+  /// The text direction to use for rendering the icon.
+  ///
+  /// If this is null, the ambient [Directionality] is used instead.
+  ///
+  /// Some icons follow the reading direction. For example, "back" buttons point
+  /// left in left-to-right environments and right in right-to-left
+  /// environments. Such icons have their [IconData.matchTextDirection] field
+  /// set to true, and the [Icon] widget uses the [textDirection] to determine
+  /// the orientation in which to draw the icon.
+  ///
+  /// This property has no effect if the [icon]'s [IconData.matchTextDirection]
+  /// field is false, but for consistency a text direction value must always be
+  /// specified, either directly using this property or using [Directionality].
+  final TextDirection textDirection;
+
   @override
   Widget build(BuildContext context) {
-    assert(debugCheckHasDirectionality(context));
-    final TextDirection textDirection = Directionality.of(context);
+    assert(this.textDirection != null || debugCheckHasDirectionality(context));
+    final TextDirection textDirection = this.textDirection ?? Directionality.of(context);
 
     final IconThemeData iconTheme = IconTheme.of(context);
 
@@ -116,6 +131,35 @@ class Icon extends StatelessWidget {
     if (iconOpacity != 1.0)
       iconColor = iconColor.withOpacity(iconColor.opacity * iconOpacity);
 
+    Widget iconWidget = new RichText(
+      textDirection: textDirection, // Since we already fetched it for the assert...
+      text: new TextSpan(
+        text: new String.fromCharCode(icon.codePoint),
+        style: new TextStyle(
+          inherit: false,
+          color: iconColor,
+          fontSize: iconSize,
+          fontFamily: icon.fontFamily,
+          package: icon.fontPackage,
+        ),
+      ),
+    );
+
+    if (icon.matchTextDirection) {
+      switch (textDirection) {
+        case TextDirection.rtl:
+          iconWidget = new Transform(
+            transform: new Matrix4.identity()..scale(-1.0),
+            alignment: Alignment.center,
+            transformHitTests: false,
+            child: iconWidget,
+          );
+          break;
+        case TextDirection.ltr:
+          break;
+      }
+    }
+
     return new Semantics(
       label: semanticLabel,
       child: new ExcludeSemantics(
@@ -123,19 +167,7 @@ class Icon extends StatelessWidget {
           width: iconSize,
           height: iconSize,
           child: new Center(
-            child: new RichText(
-              textDirection: textDirection, // Since we already fetched it for the assert...
-              text: new TextSpan(
-                text: new String.fromCharCode(icon.codePoint),
-                style: new TextStyle(
-                  inherit: false,
-                  color: iconColor,
-                  fontSize: iconSize,
-                  fontFamily: icon.fontFamily,
-                  package: icon.fontPackage,
-                ),
-              ),
-            ),
+            child: iconWidget,
           ),
         ),
       ),

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

@@ -21,6 +21,7 @@ class IconData {
     this.codePoint, {
     this.fontFamily,
     this.fontPackage,
+    this.matchTextDirection: false,
   });
 
   /// The Unicode code point at which this icon is stored in the icon font.
@@ -39,6 +40,13 @@ class IconData {
   ///  * [TextStyle], which describes how to use fonts from other packages.
   final String fontPackage;
 
+  /// Whether this icon should be automatically mirrored in right-to-left
+  /// environments.
+  ///
+  /// The [Icon] widget respects this value by mirroring the icon when the
+  /// [Directionality] is [TextDirection.rtl].
+  final bool matchTextDirection;
+
   @override
   bool operator ==(dynamic other) {
     if (runtimeType != other.runtimeType)