Add static of accessor methods to ColorScheme and TextTheme (#154073)

The most common use case to lookup a `ThemeData` instance using `Theme.of(context)` is to access either the `ColorScheme`, the `TextTheme`, or both.

Before this change:
```dart
final colors = Theme.of(context).colorScheme;
final textTheme = Theme.of(context).textTheme;
final primaryTextTheme = Theme.of(context).primaryTextTheme;
```
or
```dart
final ThemeData(
      :colorScheme,
      :textTheme,
      :primaryTextTheme,
    ) = Theme.of(context);
```
After this change:
```dart
final colors = ColorScheme.of(context);
final textTheme = TextTheme.of(context);
final primaryTextTheme = TextTheme.primaryOf(context);
```

### Primary Changes
This PR adds static `of` convenience methods to `ColorScheme` and `TextTheme` that delegate to the `ThemeData`'s respective properties. The methods added are:
* `ColorScheme.of(context)` that returns `Theme.of(context).colorScheme`.
* `TextTheme.of(context)` that returns `Theme.of(context).textTheme`.
* `TextTheme.primaryOf(context)` that returns `Theme.of(context).primaryTextTheme`.

### Side-effects
To allow the above changes to function, this PR adds:
* A `theme.dart` import to `color_scheme.dart` to access to `Theme`.
* A `theme.dart` import to `text_theme.dart` to access to `Theme`.
* A `package:flutter/widgets.dart` import to `text_theme.dart` to access `BuildContext`. 
* The above import allowed getting rid of the same `@docImport` from `text_theme.dart`.

### Documentation updates
This PR also updates the following documentation elements:
* Adds docs to the newly added members.
* Updates `TextTheme`'s docs to instruct using `TextTheme.of(context)` instead of `Theme.of(context).textTheme`.
* Updates `Theme.of` to add a "See also" section to `ColorScheme.of` and `TextTheme.of` since these use cases are among the most common ones for `Theme.of(context)`.

Fixes #72201.

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

@@ -14,7 +14,7 @@ import 'package:flutter/widgets.dart';
 import 'package:material_color_utilities/material_color_utilities.dart';
 
 import 'colors.dart';
-import 'theme_data.dart';
+import 'theme.dart';
 
 /// The algorithm used to construct a [ColorScheme] in [ColorScheme.fromSeed].
 ///
@@ -1920,4 +1920,9 @@ class ColorScheme with Diagnosticable {
       DynamicSchemeVariant.fruitSalad => SchemeFruitSalad(sourceColorHct: sourceColor, isDark: isDark, contrastLevel: contrastLevel),
     };
   }
+
+  /// The [ThemeData.colorScheme] of the ambient [Theme].
+  ///
+  /// Equivalent to `Theme.of(context).colorScheme`.
+  static ColorScheme of(BuildContext context) => Theme.of(context).colorScheme;
 }

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

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-/// @docImport 'package:flutter/widgets.dart';
-///
 /// @docImport 'elevated_button.dart';
 /// @docImport 'material.dart';
 /// @docImport 'outlined_button.dart';
@@ -13,8 +11,9 @@
 library;
 
 import 'package:flutter/foundation.dart';
-import 'package:flutter/painting.dart';
+import 'package:flutter/widgets.dart';
 
+import 'theme.dart';
 import 'typography.dart';
 
 /// Material design text theme.
@@ -23,8 +22,9 @@ import 'typography.dart';
 /// (e.g., labelLarge, bodySmall). Rather than creating a [TextTheme] directly,
 /// you can obtain an instance as [Typography.black] or [Typography.white].
 ///
-/// To obtain the current text theme, call [Theme.of] with the current
-/// [BuildContext] and read the [ThemeData.textTheme] property.
+/// To obtain the current text theme, call [TextTheme.of] with the current
+/// [BuildContext]. This is equivalent to calling [Theme.of] and reading
+/// the [ThemeData.textTheme] property.
 ///
 /// The names of the TextTheme properties match this table from the
 /// [Material Design spec](https://m3.material.io/styles/typography/tokens).
@@ -597,6 +597,25 @@ class TextTheme with Diagnosticable {
     );
   }
 
+  /// The [ThemeData.textTheme] property of the ambient [Theme].
+  ///
+  /// Equivalent to `Theme.of(context).textTheme`.
+  ///
+  /// See also:
+  /// * [TextTheme.primaryOf], which returns the [ThemeData.primaryTextTheme] property of
+  ///   the ambient [Theme] instead.
+  static TextTheme of(BuildContext context) => Theme.of(context).textTheme;
+
+  /// The [ThemeData.primaryTextTheme] property of the ambient [Theme].
+  ///
+  ///
+  /// Equivalent to `Theme.of(context).primaryTextTheme`.
+  ///
+  /// See also:
+  /// * [TextTheme.of], which returns the [ThemeData.textTheme] property of the ambient
+  ///   [Theme] instead.
+  static TextTheme primaryOf(BuildContext context) => Theme.of(context).primaryTextTheme;
+
   @override
   bool operator ==(Object other) {
     if (identical(this, other)) {

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

@@ -3,6 +3,8 @@
 // found in the LICENSE file.
 
 /// @docImport 'app.dart';
+/// @docImport 'color_scheme.dart';
+/// @docImport 'text_theme.dart';
 library;
 
 import 'package:flutter/cupertino.dart';
@@ -104,6 +106,15 @@ class Theme extends StatelessWidget {
   ///   );
   /// }
   /// ```
+  ///
+  /// See also:
+  ///
+  /// * [ColorScheme.of], a convenience method that returns [ThemeData.colorScheme]
+  ///   from the closest [Theme] ancestor. (equivalent to `Theme.of(context).colorScheme`).
+  /// * [TextTheme.of], a convenience method that returns [ThemeData.textTheme]
+  ///   from the closest [Theme] ancestor. (equivalent to `Theme.of(context).textTheme`).
+  /// * [IconTheme.of], that returns [ThemeData.iconTheme] from the closest [Theme] or
+  ///   [IconThemeData.fallback] if there is no [IconTheme] ancestor.
   static ThemeData of(BuildContext context) {
     final _InheritedTheme? inheritedTheme = context.dependOnInheritedWidgetOfExactType<_InheritedTheme>();
     final MaterialLocalizations? localizations = Localizations.of<MaterialLocalizations>(context, MaterialLocalizations);

packages/flutter/test/material/color_scheme_test.dart

@@ -901,6 +901,25 @@ void main() {
       colorsMatchDynamicSchemeColors(schemeVariant, Brightness.dark, 0.5);
     }
   });
+
+  testWidgets('ColorScheme.of(context) is equivalent to Theme.of(context).colorScheme', (WidgetTester tester) async {
+    const Key sizedBoxKey = Key('sizedBox');
+    final ColorScheme colorScheme = ColorScheme.fromSeed(seedColor: Colors.red);
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData.from(colorScheme: colorScheme),
+        home: const SizedBox(key: sizedBoxKey),
+      ),
+    );
+
+    final BuildContext context = tester.element(find.byKey(sizedBoxKey));
+    final ColorScheme colorSchemeOfTheme = Theme.of(context).colorScheme;
+    final ColorScheme colorSchemeFromContext = ColorScheme.of(context);
+
+    expect(colorSchemeOfTheme, colorScheme);
+    expect(colorSchemeFromContext, colorScheme);
+  });
 }
 
 Future<void> _testFilledButtonColor(WidgetTester tester, ColorScheme scheme, Color expectation) async {

packages/flutter/test/material/text_theme_test.dart

@@ -246,4 +246,49 @@ void main() {
     expect(fullLerp.horizontal, 2.0);
     expect(fullLerp.vertical, 1.0);
   });
+
+  testWidgets('TextTheme.of(context) is equivalent to Theme.of(context).textTheme', (WidgetTester tester) async {
+    const Key sizedBoxKey = Key('sizedBox');
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          textTheme: const TextTheme(
+            displayLarge: TextStyle(color: Colors.blue, fontSize: 30.0),
+          ),
+        ),
+        home: const SizedBox(key: sizedBoxKey),
+      ),
+    );
+    final BuildContext context = tester.element(find.byKey(sizedBoxKey));
+
+    final ThemeData themeData = Theme.of(context);
+    final TextTheme expectedTextTheme = themeData.textTheme;
+    final TextTheme actualTextTheme = TextTheme.of(context);
+
+    expect(actualTextTheme, equals(expectedTextTheme));
+
+  });
+
+  testWidgets('TextTheme.primaryOf(context) is equivalent to Theme.of(context).primaryTextTheme', (WidgetTester tester) async {
+    const Key sizedBoxKey = Key('sizedBox');
+
+    await tester.pumpWidget(
+      MaterialApp(
+        theme: ThemeData(
+          primaryTextTheme: const TextTheme(
+            displayLarge: TextStyle(backgroundColor: Colors.green, fontStyle: FontStyle.italic),
+          ),
+        ),
+        home: const SizedBox(key: sizedBoxKey),
+      ),
+    );
+
+    final BuildContext context = tester.element(find.byKey(sizedBoxKey));
+    final ThemeData themeData = Theme.of(context);
+    final TextTheme expectedTextTheme = themeData.primaryTextTheme;
+    final TextTheme actualTextTheme = TextTheme.primaryOf(context);
+
+    expect(actualTextTheme, equals(expectedTextTheme));
+  });
 }