Fix flow analysis of a late initializer that is an assignment expression

Fixes dart-lang#43621.

Bug: dart-lang#43621
Change-Id: I5847e34fba9d892085dd8e388f6d96912c46eb5a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/165682
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/definite_unassignment/data/late_initializer.dart

@@ -6,6 +6,8 @@
 // doesn't execute immediately, so it may refer to other late variables that
 // aren't assigned yet.
 
+void use(Object? x) {}
+
 eagerInitializerRefersToLateVar() {
   late int x;
   int y = /*unassigned*/ x;
@@ -17,3 +19,9 @@ lateInitializerRefersToLateVar() {
   late int y = x;
   x = 0;
 }
+
+lateInitializerIsAssignment() {
+  late int y;
+  late int z1 = y = 3;
+  use(y);
+}

pkg/front_end/lib/src/fasta/kernel/body_builder.dart

@@ -2408,11 +2408,12 @@ class BodyBuilder extends ScopeListener<JumpTarget>
     assert(assignmentOperator.stringValue == "=");
     AssignedVariablesNodeInfo<VariableDeclaration> assignedVariablesInfo;
     bool isLate = (currentLocalVariableModifiers & lateMask) != 0;
+    Expression initializer = popForValue();
     if (isLate) {
       assignedVariablesInfo = typeInferrer?.assignedVariables
           ?.deferNode(isClosureOrLateVariableInitializer: true);
     }
-    pushNewLocalVariable(popForValue(), equalsToken: assignmentOperator);
+    pushNewLocalVariable(initializer, equalsToken: assignmentOperator);
     if (isLate) {
       VariableDeclaration node = peek();
       // This is matched by the call to [beginNode] in