Add tests for lerpDouble

lib/ui/lerp.dart

@@ -7,6 +7,8 @@
 part of dart.ui;
 
 /// Linearly interpolate between two numbers.
+// TODO(cbracken): Consider making a and b non-nullable.
+//                 https://github.com/flutter/flutter/issues/64617
 double? lerpDouble(num? a, num? b, double t) {
   if (a == null && b == null)
     return null;

testing/dart/lerp_test.dart

@@ -0,0 +1,46 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// @dart = 2.10
+import 'dart:ui';
+
+import 'package:test/test.dart';
+
+void main() {
+  test('lerpDouble should return null if and only if both inputs are null', () {
+    expect(lerpDouble(null, null, 1.0), isNull);
+    expect(lerpDouble(5.0, null, 0.25), isNotNull);
+    expect(lerpDouble(null, 5.0, 0.25), isNotNull);
+  });
+
+  test('lerpDouble should treat a null input as 0 if the other input is non-null', () {
+    expect(lerpDouble(null, 10.0, 0.25), 2.5);
+    expect(lerpDouble(10.0, null, 0.25), 7.5);
+  });
+
+  test('lerpDouble should handle interpolation values < 0.0', () {
+    expect(lerpDouble(0.0, 10.0, -5.0), -50.0);
+    expect(lerpDouble(10.0, 0.0, -5.0), 60.0);
+  });
+
+  test('lerpDouble should return the start value at 0.0', () {
+    expect(lerpDouble(2.0, 10.0, 0.0), 2.0);
+    expect(lerpDouble(10.0, 2.0, 0.0), 10.0);
+  });
+
+  test('lerpDouble should interpolate between two values', () {
+    expect(lerpDouble(0.0, 10.0, 0.25), 2.5);
+    expect(lerpDouble(10.0, 0.0, 0.25), 7.5);
+  });
+
+  test('lerpDouble should return the end value at 1.0', () {
+    expect(lerpDouble(2.0, 10.0, 1.0), 10.0);
+    expect(lerpDouble(10.0, 2.0, 1.0), 2.0);
+  });
+
+  test('lerpDouble should handle interpolation values > 1.0', () {
+    expect(lerpDouble(0.0, 10.0, 5.0), 50.0);
+    expect(lerpDouble(10.0, 0.0, 5.0), -40.0);
+  });
+}