Repurpose inference-update-3 experiment for language issue 1618.

After discussion with Leaf, I believe the primary use case that
`interface-update-3` was intended to address can be better addressed
through the proposal of language issue
dart-lang/language#1618 (instead of
dart-lang/language#3471, which I had been
working on previously).

Since I had not made a lot of progress on
dart-lang/language#3471, rather than set up
a brand new experiment flag, it seems better to simply back out the
work that I'd previously done, and repurpose the `inference-update-3`
flag to work on issue 1618.

Change-Id: I6ee1cb29f722f8e1f0710cbd0600cb87b8fd26a1
Bug: dart-lang/language#1618
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/350620
Commit-Queue: Paul Berry <[email protected]>
Reviewed-by: Nate Bosch <[email protected]>
Reviewed-by: Chloe Stefantsova <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/dart/resolver/assignment_expression_resolver.dart

@@ -3,7 +3,6 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:_fe_analyzer_shared/src/flow_analysis/flow_analysis.dart';
-import 'package:analyzer/dart/analysis/features.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ -85,9 +84,7 @@ class AssignmentExpressionResolver {
     DartType? rhsContext;
     {
       var leftType = node.writeType;
-      if (writeElement is VariableElement &&
-          !_resolver.definingLibrary.featureSet
-              .isEnabled(Feature.inference_update_3)) {
+      if (writeElement is VariableElement) {
         leftType = _resolver.localVariableTypeProvider
             .getType(left as SimpleIdentifier, isRead: false);
       }

pkg/front_end/lib/src/fasta/type_inference/inference_visitor.dart

@@ -8629,8 +8629,7 @@ class InferenceVisitorImpl extends InferenceVisitorBase
     }
     DartType declaredOrInferredType = variable.lateType ?? variable.type;
     DartType? promotedType;
-    if (isNonNullableByDefault &&
-        !libraryBuilder.libraryFeatures.inferenceUpdate3.isEnabled) {
+    if (isNonNullableByDefault) {
       promotedType = flowAnalysis.promotedType(variable);
     }
     ExpressionInferenceResult rhsResult = inferExpression(

tests/language/inference_update_3/promoted_local_assignment_context_disabled_test.dart

@@ -128,8 +128,8 @@ features:
   resource-identifiers:
     help: "Output arguments used by static functions."
 
-  inference-update-3: # See https://github.com/dart-lang/language/issues/3471
-    help: "Better handling of coercions, conditional expressions, and switch expressions."
+  inference-update-3: # See https://github.com/dart-lang/language/issues/1618
+    help: "Better handling of conditional expressions, and switch expressions."
 
 # Experiment flag only used for testing.
   test-experiment: