Fix flutter in preparation for implementing Dart's "Infer non-nullability from...

Fix flutter in preparation for implementing Dart's "Infer non-nullability from local boolean variables" (#72494)

When https://github.com/dart-lang/language/issues/1274 (Infer
non-nullability from local boolean variables) is implemented, flow
analysis will detect that code like this no longer needs to perform a
null check:

    final bool contextIsValid = focus != null && focus.context != null;
    ...
    if (contextIsValid) {
      ... focus! ... // Null check unnecessary
    }

To avoid a build failure due to the unnecessary null check, we need to
temporarily write it in a way that we can ignore it.  Once the feature
is complete and rolled into flutter, I'll remove the null check
entirely.

packages/flutter/lib/src/painting/image_provider.dart

@@ -118,8 +118,6 @@ class ImageConfiguration {
     result.write('ImageConfiguration(');
     bool hasArguments = false;
     if (bundle != null) {
-      if (hasArguments)
-        result.write(', ');
       result.write('bundle: $bundle');
       hasArguments = true;
     }

packages/flutter/lib/src/widgets/scrollable.dart

@@ -983,7 +983,17 @@ class ScrollAction extends Action<ScrollIntent> {
     final bool contextIsValid = focus != null && focus.context != null;
     if (contextIsValid) {
       // Check for primary scrollable within the current context
-      if (Scrollable.of(focus!.context!) != null)
+      // After https://github.com/dart-lang/language/issues/1274 is implemented,
+      // `focus` will be promoted to non-nullable so we won't need to null check
+      // it (and it will cause a build failure to try to do so).  Until then, we
+      // need to null check it in a way that won't cause a build failure once
+      // the feature is implemented.  We can do that using an explicit "if"
+      // test.
+      // TODO(paulberry): remove this hack once the feature is implemented.
+      if (focus == null) { // ignore: dead_code
+        throw 'This throw is unreachable';
+      }
+      if (Scrollable.of(focus.context!) != null)
         return true;
       // Check for fallback scrollable with context from PrimaryScrollController
       if (PrimaryScrollController.of(focus.context!) != null) {