Add some debugging aids (#8108)

I've been using these locally, but they're likely to be useful to other people
as well.

packages/flutter/lib/src/cupertino/slider.dart

@@ -111,6 +111,14 @@ class CupertinoSlider extends StatefulWidget {
 
   @override
   _CupertinoSliderState createState() => new _CupertinoSliderState();
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('value: ${value.toStringAsFixed(1)}');
+    description.add('min: $min');
+    description.add('max: $max');
+  }
 }
 
 class _CupertinoSliderState extends State<CupertinoSlider> with TickerProviderStateMixin {

packages/flutter/lib/src/material/slider.dart

@@ -143,6 +143,14 @@ class Slider extends StatefulWidget {
 
   @override
   _SliderState createState() => new _SliderState();
+
+  @override
+  void debugFillDescription(List<String> description) {
+    super.debugFillDescription(description);
+    description.add('value: ${value.toStringAsFixed(1)}');
+    description.add('min: $min');
+    description.add('max: $max');
+  }
 }
 
 class _SliderState extends State<Slider> with TickerProviderStateMixin {

packages/flutter/lib/src/rendering/debug.dart

@@ -85,6 +85,9 @@ int debugPaintPointersColorValue = _kDebugPaintPointersColorValue;
 /// Overlay a rotating set of colors when repainting layers in checked mode.
 bool debugRepaintRainbowEnabled = false;
 
+/// Overlay a rotating set of colors when repainting text in checked mode.
+bool debugRepaintTextRainbowEnabled = false;
+
 /// The current color to overlay when repainting a layer.
 HSVColor debugCurrentRepaintColor = _kDebugCurrentRepaintColor;
 
@@ -162,6 +165,7 @@ bool debugAssertAllRenderVarsUnset(String reason) {
         debugPaintLayerBordersEnabled ||
         debugPaintPointersEnabled ||
         debugRepaintRainbowEnabled ||
+        debugRepaintTextRainbowEnabled ||
         debugPrintMarkNeedsPaintStacks ||
         debugPrintMarkNeedsLayoutStacks ||
         debugCheckIntrinsicSizes ||

packages/flutter/lib/src/rendering/object.dart

@@ -2018,6 +2018,11 @@ abstract class RenderObject extends AbstractNode implements HitTestTarget {
       parent.markNeedsPaint();
       assert(parent == this.parent);
     } else {
+      assert(() {
+        if (debugPrintMarkNeedsPaintStacks)
+          debugPrintStack(label: 'markNeedsPaint() called for $this (root of render tree)');
+        return true;
+      });
       // If we're the root of the render tree (probably a RenderView),
       // then we have to paint ourselves, since nobody else can paint
       // us. We don't add ourselves to _nodesNeedingPaint in this

packages/flutter/lib/src/rendering/paragraph.dart

@@ -7,6 +7,7 @@ import 'dart:ui' as ui;
 import 'package:flutter/gestures.dart';
 
 import 'box.dart';
+import 'debug.dart';
 import 'object.dart';
 import 'semantics.dart';
 
@@ -252,6 +253,16 @@ class RenderParagraph extends RenderBox {
     // works properly.
     _layoutTextWithConstraints(constraints);
     final Canvas canvas = context.canvas;
+
+    assert(() {
+      if (debugRepaintTextRainbowEnabled) {
+        Paint paint = new Paint()
+          ..color = debugCurrentRepaintColor.toColor();
+        canvas.drawRect(offset & size, paint);
+      }
+      return true;
+    });
+
     if (_hasVisualOverflow) {
       final Rect bounds = offset & size;
       if (_overflowShader != null)

packages/flutter/lib/src/rendering/view.dart

@@ -154,7 +154,7 @@ class RenderView extends RenderObject with RenderObjectWithChildMixin<RenderBox>
       ui.window.render(scene);
       scene.dispose();
       assert(() {
-        if (debugRepaintRainbowEnabled)
+        if (debugRepaintRainbowEnabled || debugRepaintTextRainbowEnabled)
           debugCurrentRepaintColor = debugCurrentRepaintColor.withHue(debugCurrentRepaintColor.hue + debugRepaintRainbowHueIncrement);
         return true;
       });