Deprecate CastError.

CastError is deprecated.  See
 https://github.com/dart-lang/sdk/issues/40763 for details.

Change-Id: If00963e68987a259396c4b5a0cd6d703bc7ac76c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/140885
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>

CHANGELOG.md

@@ -31,6 +31,11 @@ There are no language changes in this release.
 
 #### `dart:core`
 
+* The class `CastError` is deprecated, and all implementation specific
+    classes implementing `TypeError` or `CastError` now implement both.  In
+    a future release, `CastError` will be removed.  See issue [40763][] for
+    details.
+
 *   Adds `StackTrace.empty` constant which is the stack trace used as default
     stack trace when no better alternative is available.
 
@@ -46,6 +51,8 @@ There are no language changes in this release.
     convenient to use in-line in, for example, `=>` function bodies or
     constructor initialization lists.
 
+[40763]: https://github.com/dart-lang/sdk/issues/40763
+
 #### `dart:developer`
 
 *   The constructors for `TimelineTask` now accept an optional `filterKey`

sdk/lib/core/errors.dart

@@ -119,6 +119,7 @@ class TypeError extends Error {}
 /**
  * Error thrown by the runtime system when a cast operation fails.
  */
+@Deprecated("Use TypeError instead")
 class CastError extends Error {}
 
 /**

sdk_nnbd/lib/core/errors.dart

@@ -117,6 +117,7 @@ class TypeError extends Error {}
 /**
  * Error thrown by the runtime system when a cast operation fails.
  */
+@Deprecated("Use TypeError instead")
 class CastError extends Error {}
 
 /**