Update code in preparation for the fix to dart-lang/language#1785 to roll out.

In short, this bug prevents implicitly typed condition variables from participating in type promotion in some circumstances (see the bug for more details).  Since the bug only occurs when the condition variable is implicitly typed, we can ensure a smooth rollout by giving explicit `bool` types to the condition variables affected by this bug.  Once the fix has fully rolled out to google3 we will be able to remove these explicit types once again.

Tested:
    TAP --sample ran all affected tests and none failed
    http://test/OCL:394261565:BASE:394223559:1630517438374:b19cc4e4
PiperOrigin-RevId: 394291389

Author: stereotype441

charts_flutter/lib/src/chart_canvas.dart

@@ -150,14 +150,16 @@ class ChartCanvas implements common.ChartCanvas {
       common.Color? stroke,
       double? strokeWidthPx,
       Rectangle<num>? drawAreaBounds}) {
-    final drawStroke =
+    // TODO: remove this explicit `bool` type when no longer needed
+    // to work around https://github.com/dart-lang/language/issues/1785
+    final bool drawStroke =
         (strokeWidthPx != null && strokeWidthPx > 0.0 && stroke != null);
 
     final strokeWidthOffset = (drawStroke ? strokeWidthPx : 0);
 
     // Factor out stroke width, if a stroke is enabled.
     final fillRectBounds = new Rectangle<num>(
-        bounds.left + strokeWidthOffset! / 2,
+        bounds.left + strokeWidthOffset / 2,
         bounds.top + strokeWidthOffset / 2,
         bounds.width - strokeWidthOffset,
         bounds.height - strokeWidthOffset);
@@ -188,16 +190,15 @@ class ChartCanvas implements common.ChartCanvas {
     // [Canvas.drawRect] does not support drawing a rectangle with both a fill
     // and a stroke at this time. Use a separate rect for the stroke.
     if (drawStroke) {
-      _paint.color =
-          new Color.fromARGB(stroke!.a, stroke.r, stroke.g, stroke.b);
+      _paint.color = new Color.fromARGB(stroke.a, stroke.r, stroke.g, stroke.b);
       // Set shader to null if no draw area bounds so it can use the color
       // instead.
       _paint.shader = drawAreaBounds != null
           ? _createHintGradient(drawAreaBounds.left.toDouble(),
               drawAreaBounds.top.toDouble(), stroke)
           : null;
       _paint.strokeJoin = StrokeJoin.round;
-      _paint.strokeWidth = strokeWidthPx!;
+      _paint.strokeWidth = strokeWidthPx;
       _paint.style = PaintingStyle.stroke;
 
       canvas.drawRect(_getRect(bounds), _paint);