Fix typos (#18017)

packages/flutter/lib/src/foundation/diagnostics.dart

@@ -17,7 +17,7 @@ enum DiagnosticLevel {
   ///
   /// If a user chooses to display [hidden] diagnostics, they should not expect
   /// the diagnostics to be formatted consistently with other diagnostics and
-  /// they should expect them to sometimes be be misleading. For example,
+  /// they should expect them to sometimes be misleading. For example,
   /// [FlagProperty] and [ObjectFlagProperty] have uglier formatting when the
   /// property `value` does does not match a value with a custom flag
   /// description. An example of a misleading diagnostic is a diagnostic for
@@ -2156,7 +2156,7 @@ abstract class Diagnosticable {
   ///    uninteresting. For example, specify a default value of null any time
   ///    a property being null does not indicate an error.
   ///  * Avoid specifying the `level` parameter unless the result you want
-  ///    cannot be be achieved by using the `defaultValue` parameter or using
+  ///    cannot be achieved by using the `defaultValue` parameter or using
   ///    the [ObjectFlagProperty] class to conditionally display the property
   ///    as a flag.
   ///  * Specify `showName` and `showSeparator` in rare cases where the string

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

@@ -63,7 +63,7 @@ class BottomSheet extends StatefulWidget {
 
   /// Called when the bottom sheet begins to close.
   ///
-  /// A bottom sheet might be be prevented from closing (e.g., by user
+  /// A bottom sheet might be prevented from closing (e.g., by user
   /// interaction) even after this callback is called. For this reason, this
   /// callback might be call multiple times for a given bottom sheet.
   final VoidCallback onClosing;

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

@@ -47,7 +47,7 @@ class Checkbox extends StatefulWidget {
   /// The following arguments are required:
   ///
   /// * [value], which determines whether the checkbox is checked. The [value]
-  ///   can only be be null if [tristate] is true.
+  ///   can only be null if [tristate] is true.
   /// * [onChanged], which is called when the value of the checkbox should
   ///   change. It can be set to null to disable the checkbox.
   ///