Add tests for lerpDouble (flutter/engine#20778)

The behaviour of lerpDouble with respect to null inputs isn't entirely
obvious. In the case where both inputs are null, it returns null.
Otherwise, it defaults the null parameter to 0.0 and carries on.

Post non-null by default, it might be nice to strengthen the parameter
contract to require them to be non-null. While this would be a breaking
change, it seems likely that the framework either meets this guarantee
or can provide it without a framework breaking change.

https://github.com/flutter/flutter/issues/64617 tracks the above.

In the meantime, adding a test to lock in the current behaviour.

engine/src/flutter/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;

engine/src/flutter/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);
+  });
+}