Add missing @SInCE markers on unmodifiable set additions.

Change-Id: I84e94736a425d45b6b1334e90f7991fa31070713
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/176240
Commit-Queue: Lasse R.H. Nielsen <[email protected]>
Reviewed-by: Kevin Moore <[email protected]>
Auto-Submit: Lasse R.H. Nielsen <[email protected]>

Author: lrhn

pkg/dev_compiler/tool/dartdevc_nnbd_sdk_error_golden.txt

@@ -1,11 +1,11 @@
 ERROR|COMPILE_TIME_ERROR|CONST_CONSTRUCTOR_THROWS_EXCEPTION|lib/core/core.dart|3679|5|94|Const constructors can't throw exceptions.
-ERROR|COMPILE_TIME_ERROR|CONST_CONSTRUCTOR_THROWS_EXCEPTION|lib/core/core.dart|7887|5|97|Const constructors can't throw exceptions.
+ERROR|COMPILE_TIME_ERROR|CONST_CONSTRUCTOR_THROWS_EXCEPTION|lib/core/core.dart|7888|5|97|Const constructors can't throw exceptions.
 ERROR|COMPILE_TIME_ERROR|CONST_CONSTRUCTOR_THROWS_EXCEPTION|lib/core/core.dart|893|5|95|Const constructors can't throw exceptions.
 ERROR|COMPILE_TIME_ERROR|CONST_CONSTRUCTOR_THROWS_EXCEPTION|lib/core/core.dart|926|5|94|Const constructors can't throw exceptions.
 ERROR|COMPILE_TIME_ERROR|INVALID_ASSIGNMENT|lib/_internal/js_dev_runtime/private/interceptors.dart|1358|18|27|A value of type 'double' can't be assigned to a variable of type 'int'.
 ERROR|COMPILE_TIME_ERROR|RETURN_OF_INVALID_TYPE|lib/_internal/js_dev_runtime/private/interceptors.dart|1225|14|38|A value of type 'double' can't be returned from method '%' because it has a return type of 'JSNumber'.
 ERROR|COMPILE_TIME_ERROR|RETURN_OF_INVALID_TYPE|lib/_internal/js_dev_runtime/private/interceptors.dart|1227|14|38|A value of type 'double' can't be returned from method '%' because it has a return type of 'JSNumber'.
 ERROR|SYNTACTIC_ERROR|CONST_FACTORY|lib/core/core.dart|3677|3|5|Only redirecting factory constructors can be declared to be 'const'.
-ERROR|SYNTACTIC_ERROR|CONST_FACTORY|lib/core/core.dart|7885|3|5|Only redirecting factory constructors can be declared to be 'const'.
+ERROR|SYNTACTIC_ERROR|CONST_FACTORY|lib/core/core.dart|7886|3|5|Only redirecting factory constructors can be declared to be 'const'.
 ERROR|SYNTACTIC_ERROR|CONST_FACTORY|lib/core/core.dart|891|3|5|Only redirecting factory constructors can be declared to be 'const'.
 ERROR|SYNTACTIC_ERROR|CONST_FACTORY|lib/core/core.dart|924|3|5|Only redirecting factory constructors can be declared to be 'const'.

sdk/lib/collection/set.dart

@@ -400,6 +400,7 @@ class _UnmodifiableSet<E> extends _SetBase<E> with _UnmodifiableSetMixin<E> {
 ///
 /// Methods that could change the set, such as [add] and [remove],
 /// must not be called.
+@Since("2.12")
 class UnmodifiableSetView<E> extends SetBase<E> with _UnmodifiableSetMixin<E> {
   final Set<E> _source;
 

sdk/lib/core/set.dart

@@ -96,6 +96,7 @@ abstract class Set<E> extends EfficientLengthIterable<E> {
    * The new set behaves like the result of [Set.of],
    * except that the set returned by this constructor is not modifiable.
    */
+  @Since("2.12")
   factory Set.unmodifiable(Iterable<E> elements) =>
       UnmodifiableSetView<E>(<E>{...elements});