[ui] Add null check in FontWeight.lerp (flutter/engine#8274)

* Add null check in FontWeight.lerp

* Add Unit tests for FontWeight.lerp

engine/src/flutter/lib/ui/text.dart

@@ -63,7 +63,8 @@ class FontWeight {
   /// Rather than using fractional weights, the interpolation rounds to the
   /// nearest weight.
   ///
-  /// Any null values for `a` or `b` are interpreted as equivalent to [normal]
+  /// If both `a` and `b` are null, then this method will return null. Otherwise,
+  /// any null values for `a` or `b` are interpreted as equivalent to [normal]
   /// (also known as [w400]).
   ///
   /// The `t` argument represents position on the timeline, with 0.0 meaning
@@ -80,6 +81,8 @@ class FontWeight {
   /// an [AnimationController].
   static FontWeight lerp(FontWeight a, FontWeight b, double t) {
     assert(t != null);
+    if (a == null && b == null)
+      return null;
     return values[lerpDouble(a?.index ?? normal.index, b?.index ?? normal.index, t).round().clamp(0, 8)];
   }
 

engine/src/flutter/testing/dart/text_test.dart

@@ -0,0 +1,27 @@
+// Copyright 2019 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.
+
+import 'dart:ui';
+
+import 'package:test/test.dart';
+
+void main() {
+  group('FontWeight.lerp', () {
+    test('works with non-null values', () {
+      expect(FontWeight.lerp(FontWeight.w400, FontWeight.w600, .5), equals(FontWeight.w500));
+    });
+
+    test('returns null if a and b are null', () {
+      expect(FontWeight.lerp(null, null, 0), isNull);
+    });
+
+    test('returns FontWeight.w400 if a is null', () {
+      expect(FontWeight.lerp(null, FontWeight.w400, 0), equals(FontWeight.w400));
+    });
+
+    test('returns FontWeight.w400 if b is null', () {
+      expect(FontWeight.lerp(FontWeight.w400, null, 1), equals(FontWeight.w400));
+    });
+  });
+}