null-annotate lerp.dart

Author: yjbanov

lib/ui/lerp.dart

@@ -6,7 +6,7 @@
 part of dart.ui;
 
 /// Linearly interpolate between two numbers.
-double lerpDouble(num a, num b, double t) {
+double/*!*/ lerpDouble(num/*?*/ a, num/*?*/ b, double/*!*/ t) {
   if (a == null && b == null)
     return null;
   a ??= 0.0;