Resolving CI failures in web (#973)

web/charts/lib/behaviors/slider.dart

@@ -17,7 +17,6 @@ import 'dart:math';
 // EXCLUDE_FROM_GALLERY_DOCS_END
 import 'package:charts_flutter/flutter.dart' as charts;
 import 'package:flutter/material.dart';
-import 'package:flutter/rendering.dart';
 import 'package:flutter/scheduler.dart';
 
 /// This is just a simple line chart with a behavior that adds slider controls.

web/github_dataviz/lib/constants.dart

@@ -1,5 +1,4 @@
 import 'package:flutter/material.dart';
-import 'dart:ui';
 
 class Constants {
   static const Color backgroundColor = Color(0xFF000020);

web/github_dataviz/lib/layered_chart.dart

@@ -1,8 +1,6 @@
 import 'dart:math';
 
 import 'package:flutter/material.dart';
-import 'package:flutter/painting.dart';
-import 'package:flutter/widgets.dart';
 import 'package:github_dataviz/catmull.dart';
 import 'package:github_dataviz/constants.dart';
 import 'package:github_dataviz/data/data_series.dart';