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:
    Some test failures are present, but the CL author has decided to mail the change anyway
PiperOrigin-RevId: 394482159

Author: stereotype441

charts_common/lib/src/chart/bar/base_bar_renderer.dart

@@ -398,8 +398,11 @@ abstract class BaseBarRenderer<D, R extends BaseBarRendererElement,
         final domainValue = domainFn(barIndex);
 
         final measureValue = measureFn(barIndex);
-        final measureIsNull = measureValue == null;
-        final measureIsNegative = !measureIsNull && measureValue! < 0;
+        // TODO: remove this explicit `bool` type when no longer
+        // needed to work around
+        // https://github.com/dart-lang/language/issues/1785
+        final bool measureIsNull = measureValue == null;
+        final measureIsNegative = !measureIsNull && measureValue < 0;
 
         // Each bar should be stored in barStackMap in a structure that mirrors
         // the visual rendering of the bars. Thus, they should be grouped by

charts_common/lib/src/chart/common/behavior/selection/select_nearest.dart

@@ -245,7 +245,9 @@ class SelectNearest<D> implements ChartBehavior<D> {
       final domainFn = series.domainFn;
       final domainLowerBoundFn = series.domainLowerBoundFn;
       final domainUpperBoundFn = series.domainUpperBoundFn;
-      final testBounds =
+      // TODO: remove this explicit `bool` type when no longer
+      // needed to work around https://github.com/dart-lang/language/issues/1785
+      final bool testBounds =
           domainLowerBoundFn != null && domainUpperBoundFn != null;
 
       for (var i = 0; i < series.data.length; i++) {
@@ -260,8 +262,8 @@ class SelectNearest<D> implements ChartBehavior<D> {
         if (domain == nearestDomain) {
           data.add(SeriesDatum(series, datum));
         } else if (testBounds) {
-          final domainLowerBound = domainLowerBoundFn!(i);
-          final domainUpperBound = domainUpperBoundFn!(i);
+          final domainLowerBound = domainLowerBoundFn(i);
+          final domainUpperBound = domainUpperBoundFn(i);
 
           var addDatum = false;
           if (domainLowerBound != null && domainUpperBound != null) {