[analyzer] Add Sink and Socket to mock_sdk

This is necessary in order to write unit tests for the `close_sinks`
linter rule.

Change-Id: I1d7d122bf76b87be92f45970fb338d861afd52e1
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279327
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Phil Quitslund <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

pkg/analyzer/lib/src/test_utilities/mock_sdk.dart

@@ -363,6 +363,17 @@ class Duration implements Comparable<Duration> {
   int compareTo(Duration other) => 0;
 }
 
+abstract class Enum {
+  int get index; // Enum
+  String get _name;
+}
+
+abstract class _Enum implements Enum {
+  final int index;
+  final String _name;
+  const _Enum(this.index, this._name);
+}
+
 class Error {
   Error();
   static String safeToString(Object? object) => '';
@@ -567,17 +578,6 @@ class Object {
   static int hashAllUnordered(Iterable<Object?> objects) => 0;
 }
 
-abstract class Enum {
-  int get index; // Enum
-  String get _name;
-}
-
-abstract class _Enum implements Enum {
-  final int index;
-  final String _name;
-  const _Enum(this.index, this._name);
-}
-
 abstract class Pattern {
   Iterable<Match> allMatches(String string, [int start = 0]);
 }
@@ -610,6 +610,10 @@ abstract class Set<E> implements Iterable<E> {
       throw '';
 }
 
+abstract class Sink {
+  void close();
+}
+
 class StackTrace {}
 
 abstract class String implements Comparable<String>, Pattern {
@@ -1231,6 +1235,10 @@ abstract class Process {
     Encoding? stderrEncoding,
   });
 }
+
+abstract class Socket {
+  void destroy() {}
+}
 ''',
     )
   ],