Update web/ samples to work with Flutter SDK (#134)

* add package:http dependency in dad_jokes

* add package:http dependency in filipino_cuisine

* don't build package:http demos until flutter/flutter#34858 is resolved

* update gallery

* update github_dataviz

* update particle_background

* don't build github_dataviz (uses package:http)

* update slide_puzzle

* update spinning_square

* update timeflow

* update vision_challenge

* update charts

* update dad_jokes

* update filipino cuisine

* ignore build output

* update timeflow and vision_challenge

* update slide_puzzle

* don't commit build/ directory

* move preview.png images to assets

* fix path url join

* update readme

* update web/readme.md

web/slide_puzzle/lib/src/frame_nanny.dart

@@ -0,0 +1,63 @@
+import 'dart:collection';
+
+class FrameNanny {
+  static const _bufferSize = 200;
+  static const _maxFrameDuration = Duration(milliseconds: 34);
+  final _buffer = ListQueue<Duration>(_bufferSize);
+  final _watch = Stopwatch();
+
+  Duration tick(Duration source) {
+    _watch.start();
+    _buffer.add(source);
+
+    while (_buffer.length > _bufferSize) {
+      _buffer.removeFirst();
+    }
+
+    if (source > _maxFrameDuration) {
+      source = _maxFrameDuration;
+    }
+
+    if (_watch.elapsed > const Duration(seconds: 2)) {
+      var goodCount = 0;
+      var sum = const Duration();
+      Duration best, worst;
+
+      for (var e in _buffer) {
+        sum += e;
+        if (e <= _maxFrameDuration) {
+          goodCount++;
+        }
+
+        if (best == null || e < best) {
+          best = e;
+        }
+
+        if (worst == null || e > worst) {
+          worst = e;
+        }
+      }
+
+      _watch.reset();
+      print([
+        '**Nanny**',
+        '${(100 * goodCount / _buffer.length).toStringAsFixed(1)}%',
+        '<= ${_maxFrameDuration.inMilliseconds}ms',
+        'best:',
+        best?.inMilliseconds,
+        'avg:',
+        _safeDivide(sum, _buffer.length),
+        'worst',
+        worst?.inMilliseconds
+      ].join(' '));
+    }
+    return source;
+  }
+}
+
+Object _safeDivide(Duration source, int divisor) {
+  if (divisor == 0) {
+    return double.nan;
+  }
+  return (source ~/ divisor).inMilliseconds;
+}