Adds ColorSwatch matcher (#155272)

https://github.com/flutter/flutter/issues/155113

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packages/flutter_test/lib/src/matchers.dart

@@ -221,11 +221,27 @@ const Matcher isInCard = _IsInCard();
 ///  * [isInCard], the opposite.
 const Matcher isNotInCard = _IsNotInCard();
 
+/// Default threshold for [isSameColorAs] and [isSameColorSwatchAs].
+const double colorEpsilon = 0.004;
+
+/// Asserts that the object represents the same color swatch as [color] when
+/// used to paint.
+///
+/// Specifically this matcher checks the object is of type [ColorSwatch] and its
+/// color components fall below the delta specified by [threshold].
+///
+/// Note: This doesn't recurse into the swatches [Color] type, instead treating
+/// them as [Color]s.
+Matcher isSameColorSwatchAs<T>(ColorSwatch<T> color,
+    {double threshold = colorEpsilon}) {
+  return _ColorSwatchMatcher<T>(color, threshold);
+}
+
 /// Asserts that the object represents the same color as [color] when used to paint.
 ///
 /// Specifically this matcher checks the object is of type [Color] and its color
 /// components fall below the delta specified by [threshold].
-Matcher isSameColorAs(Color color, {double threshold = 0.004}) {
+Matcher isSameColorAs(Color color, {double threshold = colorEpsilon}) {
   return _ColorMatcher(color, threshold);
 }
 
@@ -2138,6 +2154,39 @@ class _CoversSameAreaAs extends Matcher {
     description.add('covers expected area and only expected area');
 }
 
+class _ColorSwatchMatcher<T> extends Matcher {
+  _ColorSwatchMatcher(this._target, this._threshold);
+
+  final ColorSwatch<T> _target;
+  final double _threshold;
+
+  @override
+  Description describe(Description description) {
+    return description.add('matches color swatch "$_target" with threshold "$_threshold".');
+  }
+
+  @override
+  bool matches(dynamic item, Map<dynamic, dynamic> matchState) {
+    if (item is ColorSwatch) {
+      final _ColorMatcher matcher = _ColorMatcher(_target, _threshold);
+      if (!matcher.matches(item, matchState)) {
+        return false;
+      }
+
+      for (final T key in _target.keys) {
+        final _ColorMatcher matcher = _ColorMatcher(_target[key]!, _threshold);
+        if (!matcher.matches(item[key], matchState)) {
+          return false;
+        }
+      }
+
+      return item.keys.length == _target.keys.length;
+    } else {
+      return false;
+    }
+  }
+}
+
 class _ColorMatcher extends Matcher {
   _ColorMatcher(this._target, this._threshold);
 

packages/flutter_test/test/matchers_test.dart

@@ -352,6 +352,25 @@ void main() {
     );
   });
 
+  test('isSameColorSwatchAs', () {
+    expect(
+      const ColorSwatch<String>(0xaaaaaaaa,
+          <String, Color>{'foo': Color(0xaaaaaaaa), 'bar': Color(0xbbbbbbbb)}),
+      isSameColorSwatchAs(const ColorSwatch<String>(0xaaaaaaaa,
+          <String, Color>{'foo': Color(0xaaaaaaaa), 'bar': Color(0xbbbbbbbb)})),
+    );
+
+    expect(
+      const ColorSwatch<String>(0xaaaaaaaa,
+          <String, Color>{'foo': Color(0xaaaaaaaa), 'bar': Color(0xbbbbbbbb)}),
+      isNot(isSameColorSwatchAs(const ColorSwatch<String>(
+          0xaaaaaaaa, <String, Color>{
+        'foo': Color(0xaaaaaaaa),
+        'bar': Color(0xcccccccc)
+      }))),
+    );
+  });
+
   test('isSameColorAs', () {
     expect(
       const Color(0x87654321),