Mark new IndexError methods with @Since

The new methods `IndexError.withLength` and `IndexError.check`
shouldn't be used by any projects relying on SDK versions prior to
2.19 (since they're not available in older SDKs).  Adding
`@Since("2.19")` ensures that once
#34978 is fixed, these methods
will trigger a lint to fire if they're used in a project targeting an
older SDK version.

Change-Id: I5b43ffba2def391b84a4f3f7d8eecaae766e7656
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/269142
Reviewed-by: Kallen Tu <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

sdk/lib/core/errors.dart

@@ -437,6 +437,7 @@ class IndexError extends ArgumentError implements RangeError {
   /// The [length] is the length of [indexable] at the time of the error.
   ///
   /// The message is used as part of the string representation of the error.
+  @Since("2.19")
   IndexError.withLength(int invalidValue, this.length,
       {this.indexable, String? name, String? message})
       : super.value(invalidValue, name, message ?? "Index out of range");
@@ -459,6 +460,7 @@ class IndexError extends ArgumentError implements RangeError {
   /// The [message], if provided, is included in the error string.
   ///
   /// Returns [index] if it is a valid index.
+  @Since("2.19")
   static int check(int index, int length,
       {Object? indexable, String? name, String? message}) {
     // Comparing with `0` as receiver produces better dart2js type inference.