[Material] Create TabBarTheme.of constructor (#23702)

* Create TabBarTheme of(...) constructor

* First round comments

* fix imports

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

@@ -4,8 +4,10 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart';
 
 import 'tabs.dart';
+import 'theme.dart';
 
 /// Defines a theme for [TabBar] widgets.
 ///
@@ -13,9 +15,8 @@ import 'tabs.dart';
 /// the [TabBar.indicator].
 ///
 /// Descendant widgets obtain the current theme's [TabBarTheme] object using
-/// `Theme.of(context).tabBarTheme`.
-/// [ThemeData.tabBarTheme] can be customized by copying it (using
-/// [TabBarTheme.copyWith]).
+/// `TabBarTheme.of(context)`. Instances of [TabBarTheme] can be customized with
+/// [TabBarTheme.copyWith].
 ///
 /// See also:
 ///
@@ -59,6 +60,11 @@ class TabBarTheme extends Diagnosticable {
     );
   }
 
+  /// The data from the closest [TabBarTheme] instance given the build context.
+  static TabBarTheme of(BuildContext context) {
+    return Theme.of(context).tabBarTheme;
+  }
+
   /// Linearly interpolate between two tab bar themes.
   ///
   /// The arguments must not be null.

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

@@ -150,7 +150,7 @@ class _TabStyle extends AnimatedWidget {
   @override
   Widget build(BuildContext context) {
     final ThemeData themeData = Theme.of(context);
-    final TabBarTheme tabBarTheme = themeData.tabBarTheme;
+    final TabBarTheme tabBarTheme = TabBarTheme.of(context);
 
     final TextStyle defaultStyle = labelStyle ?? themeData.primaryTextTheme.body2;
     final TextStyle defaultUnselectedStyle = unselectedLabelStyle ?? labelStyle ?? themeData.primaryTextTheme.body2;
@@ -513,7 +513,7 @@ class _TabBarScrollController extends ScrollController {
 ///
 /// Requires one of its ancestors to be a [Material] widget.
 ///
-/// Uses values from [ThemeData.tabBarTheme] if it is set in the current context.
+/// Uses values from [TabBarTheme] if it is set in the current context.
 ///
 /// See also:
 ///
@@ -698,11 +698,11 @@ class _TabBarState extends State<TabBar> {
   Decoration get _indicator {
     if (widget.indicator != null)
       return widget.indicator;
-    final ThemeData themeData = Theme.of(context);
-    if (themeData.tabBarTheme.indicator != null)
-      return themeData.tabBarTheme.indicator;
+    final TabBarTheme tabBarTheme = TabBarTheme.of(context);
+    if (tabBarTheme.indicator != null)
+      return tabBarTheme.indicator;
 
-    Color color = widget.indicatorColor ?? themeData.indicatorColor;
+    Color color = widget.indicatorColor ?? Theme.of(context).indicatorColor;
     // ThemeData tries to avoid this by having indicatorColor avoid being the
     // primaryColor. However, it's possible that the tab bar is on a
     // Material that isn't the primaryColor. In that case, if the indicator
@@ -755,7 +755,7 @@ class _TabBarState extends State<TabBar> {
     _indicatorPainter = _controller == null ? null : _IndicatorPainter(
       controller: _controller,
       indicator: _indicator,
-      indicatorSize: widget.indicatorSize ?? Theme.of(context).tabBarTheme.indicatorSize,
+      indicatorSize: widget.indicatorSize ?? TabBarTheme.of(context).indicatorSize,
       tabKeys: _tabKeys,
       old: _indicatorPainter,
     );