stream_colors [nfc]: Make StreamColorSwatches.lerp static

Discussion:
  #746 (comment)

lib/widgets/stream_colors.dart

@@ -23,25 +23,25 @@ abstract class StreamColorSwatches {
 
   StreamColorSwatch _computeForBaseColor(int base);
 
-  /// Gives a [StreamColorSwatches], lerped to [other] at [t].
+  /// Gives a [StreamColorSwatches], lerped between [a] and [b] at [t].
   ///
-  /// If [this] and [other] are [identical], returns [this].
+  /// If [a] and [b] are [identical], returns [this].
   ///
   /// Else returns an instance whose [forBaseColor] will call
-  /// [this.forBaseColor] and [other.forBaseColor]
+  /// [a.forBaseColor] and [b.forBaseColor]
   /// and return [StreamColorSwatch.lerp]'s result on those.
   /// This computation is cached on the instance
   /// in order to save work building [t]'s animation frame when there are
   /// multiple UI elements using the same [subscription.color].
-  StreamColorSwatches lerp(StreamColorSwatches other, double t) {
+  static StreamColorSwatches lerp(StreamColorSwatches a, StreamColorSwatches b, double t) {
     // This short-circuit helps when [a] and [b]
     // are both [StreamColorSwatches.light]
     // or both [StreamColorSwatches.dark].
     // Empirically, [lerp] is called even when the theme hasn't changed,
     // so this is an important optimization.
-    if (identical(this, other)) return this;
+    if (identical(a, b)) return a;
 
-    return _StreamColorSwatchesLerped(this, other, t);
+    return _StreamColorSwatchesLerped(a, b, t);
   }
 }
 

lib/widgets/theme.dart

@@ -141,7 +141,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       borderBar: Color.lerp(borderBar, other.borderBar, t)!,
       icon: Color.lerp(icon, other.icon, t)!,
       title: Color.lerp(title, other.title, t)!,
-      streamColorSwatches: streamColorSwatches.lerp(other.streamColorSwatches, t),
+      streamColorSwatches: StreamColorSwatches.lerp(streamColorSwatches, other.streamColorSwatches, t),
     );
   }
 }

test/widgets/stream_colors_test.dart

@@ -40,17 +40,16 @@ void main() {
 
     group('lerp', () {
       test('on identical instances', () {
-        check(
-          StreamColorSwatches.light.lerp(StreamColorSwatches.light, 0.5)
-        ).identicalTo(StreamColorSwatches.light);
+        final light = StreamColorSwatches.light;
+        check(StreamColorSwatches.lerp(light, light, 0.5)).identicalTo(light);
 
-        check(
-          StreamColorSwatches.dark.lerp(StreamColorSwatches.dark, 0.5)
-        ).identicalTo(StreamColorSwatches.dark);
+        final dark = StreamColorSwatches.dark;
+        check(StreamColorSwatches.lerp(dark, dark, 0.5)).identicalTo(dark);
       });
 
       test('from light to dark', () {
-        final instance = StreamColorSwatches.light.lerp(StreamColorSwatches.dark, 0.4);
+        final instance = StreamColorSwatches
+          .lerp(StreamColorSwatches.light, StreamColorSwatches.dark, 0.4);
 
         const base1 = 0xff76ce90;
         final swatch1 = instance.forBaseColor(base1);