[flow] Add unit test for postIncDec().

Add Flow Analysis unit tests for FlowAnalysis.postIncDec() which should be used for postfix increment and decrement operations.

Bug: dart-lang/language#3658
Change-Id: If77b6fbb5fc80d5f5d014ec0516d77578446dced
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393441
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Kallen Tu <kallentu@google.com>

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_test.dart

@@ -2154,6 +2154,27 @@ main() {
       ]);
     });
 
+    test('postIncDec() does not store expressionInfo in the write', () {
+      // num x;
+      // if (x++ is int) {
+      //   x is not promoted.
+      // }
+
+      var x = Var('x');
+      h.run([
+        declare(x, type: 'num'),
+        if_(
+            x
+                .postIncDec()
+                .is_('int')
+                .getExpressionInfo((info) => expect(info, isNull)),
+            [
+              checkNotPromoted(x),
+            ]),
+        checkNotPromoted(x),
+      ]);
+    });
+
     test('switchExpression throw in scrutinee makes all cases unreachable', () {
       h.run([
         switchExpr(throw_(expr('C')), [

pkg/_fe_analyzer_shared/test/mini_ast.dart

@@ -2529,6 +2529,9 @@ abstract class LValue extends Expression {
         location: location);
   }
 
+  void _visitPostIncDec(
+      Harness h, Expression postIncDecExpression, Type writtenType);
+
   void _visitWrite(Harness h, Expression assignmentExpression, Type writtenType,
       Expression? rhs);
 }
@@ -3851,6 +3854,31 @@ mixin PossiblyGuardedPattern on Node implements ProtoSwitchHead {
   }
 }
 
+/// Representation of a postfix increment or decrement operation.
+class PostIncDec extends Expression {
+  final LValue lhs;
+
+  PostIncDec(this.lhs, {required super.location});
+
+  @override
+  void preVisit(PreVisitor visitor) {
+    lhs.preVisit(visitor, disposition: _LValueDisposition.readWrite);
+  }
+
+  @override
+  String toString() => '$lhs++';
+
+  @override
+  ExpressionTypeAnalysisResult<Type> visit(
+      Harness h, SharedTypeSchemaView<Type> schema) {
+    Type type = h.typeAnalyzer
+        .analyzeExpression(lhs, h.operations.unknownType)
+        .unwrapTypeView();
+    lhs._visitPostIncDec(h, this, type);
+    return new SimpleTypeAnalysisResult<Type>(type: SharedTypeView(type));
+  }
+}
+
 /// Data structure holding information needed during the "pre-visit" phase of
 /// type analysis.
 class PreVisitor {
@@ -3916,6 +3944,12 @@ class Property extends PromotableLValue {
         ?.unwrapTypeView();
   }
 
+  @override
+  void _visitPostIncDec(
+      Harness h, Expression postIncDecExpression, Type writtenType) {
+    // No flow analysis impact
+  }
+
   @override
   void _visitWrite(Harness h, Expression assignmentExpression, Type writtenType,
       Expression? rhs) {
@@ -4673,6 +4707,12 @@ class ThisOrSuperProperty extends PromotableLValue {
         ?.unwrapTypeView();
   }
 
+  @override
+  void _visitPostIncDec(
+      Harness h, Expression postIncDecExpression, Type writtenType) {
+    // No flow analysis impact
+  }
+
   @override
   void _visitWrite(Harness h, Expression assignmentExpression, Type writtenType,
       Expression? rhs) {
@@ -4856,6 +4896,16 @@ class Var extends Node
   @override
   void preVisit(PreVisitor visitor) {}
 
+  /// Creates an expression representing a postfix increment or decrement
+  /// operation applied to this variable.
+  Expression postIncDec() {
+    var location = computeLocation();
+    return new PostIncDec(
+      new VariableReference._(this, null, location: location),
+      location: location,
+    );
+  }
+
   /// Creates an expression representing a read of this variable, which as a
   /// side effect will call the given callback with the returned promoted type.
   Expression readAndCheckPromotedType(void Function(Type?) callback) =>
@@ -4978,6 +5028,16 @@ class VariableReference extends LValue {
     return result;
   }
 
+  @override
+  void _visitPostIncDec(
+      Harness h, Expression postIncDecExpression, Type writtenType) {
+    h.flow.postIncDec(
+      postIncDecExpression,
+      variable,
+      SharedTypeView(writtenType),
+    );
+  }
+
   @override
   void _visitWrite(Harness h, Expression assignmentExpression, Type writtenType,
       Expression? rhs) {