Fix passing "void" into "dynamic" to future-proof new changes. (#14366)

Using a void list literal in this case is valid and correct, and will be
required for dart 2.

Note: void can be inferred here, but followed the lints to write it
explicitly even though that's arguably a bit weird in this case and less
backwards compatible.

packages/flutter/test/material/animated_icons_test.dart

@@ -161,7 +161,7 @@ void main() {
     final CustomPaint customPaint = tester.widget(find.byType(CustomPaint));
     final MockCanvas canvas = new MockCanvas();
     customPaint.painter.paint(canvas, const Size(48.0, 48.0));
-    verifyInOrder(<dynamic>[
+    verifyInOrder(<void>[
       canvas.rotate(math.pi),
       canvas.translate(-48.0, -48.0)
     ]);
@@ -208,7 +208,7 @@ void main() {
     final CustomPaint customPaint = tester.widget(find.byType(CustomPaint));
     final MockCanvas canvas = new MockCanvas();
     customPaint.painter.paint(canvas, const Size(48.0, 48.0));
-    verifyInOrder(<dynamic>[
+    verifyInOrder(<void>[
       canvas.rotate(math.pi),
       canvas.translate(-48.0, -48.0)
     ]);