[dart:io] Provide epoch timestamps in the FileStat notFound object.

This is a breaking change. #40706

The dummy object returned if FileStat.stat() and FileStat.statSync()
fail now contains Unix epoch timestamps instead of null for the
accessed, changed, and modified getters.

These timestamps are always non-null if the API succeeds and the
timestamps are meaningless when the API fails and returns the
FileSystemEntityType.notFound type. This change makes the timestamps
always non-null, which avoids all legitimate accesses needing a needless
null check when Dart becomes null safe. This change is consistent with
the mode and size getters that are initialized to non-null dummy values
when the API fail.

The NNBD migration required making subtle changes to some dart:io
semantics in order to provide a better API. This change backports one of
these semantic changes to the unmigrated SDK so any issues can be
discovered now instead of blocking the future SDK unfork.

Change-Id: Iff6b34d04b60f4c9f4cf8d9dd0679f721d142ba4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/136585
Reviewed-by: Lasse R.H. Nielsen <[email protected]>

Author: sortie

CHANGELOG.md

@@ -61,6 +61,11 @@ used (see Issue [39627][]).
   `ProcessStartMode.inheritStdio`) upon accessing the `stdin`, `stdout`, and
   `stderr` getters. Previously these getters would all return `null`.
 
+* **Breaking change** [#40706](https://github.com/dart-lang/sdk/issues/40706):
+  The dummy object returned if `FileStat.stat()` and `FileStat.statSync()` fail
+  now contains Unix epoch timestamps instead of `null` for the `accessed`,
+  `changed`, and `modified` getters.
+
 #### `dart:mirrors`
 
 * Added `MirrorSystem.neverType`.

sdk/lib/io/file_system_entity.dart

@@ -59,7 +59,9 @@ class FileStat {
   static const _mode = 4;
   static const _size = 5;
 
-  static const _notFound = const FileStat._internalNotFound();
+  static final _epoch = DateTime.fromMillisecondsSinceEpoch(0, isUtc: true);
+  static final _notFound = new FileStat._internal(
+      _epoch, _epoch, _epoch, FileSystemEntityType.notFound, 0, -1);
 
   /**
    * The time of the last change to the data or metadata of the file system
@@ -105,14 +107,6 @@ class FileStat {
   FileStat._internal(this.changed, this.modified, this.accessed, this.type,
       this.mode, this.size);
 
-  const FileStat._internalNotFound()
-      : changed = null,
-        modified = null,
-        accessed = null,
-        type = FileSystemEntityType.notFound,
-        mode = 0,
-        size = -1;
-
   external static _statSync(_Namespace namespace, String path);
 
   /**