[flow analysis] Address style comments from recent code reviews.

This CL addresses style comments from Lasse on
https://dart-review.googlesource.com/c/sdk/+/429227 and
https://dart-review.googlesource.com/c/sdk/+/431740.

Change-Id: I87add7fc6179e186545aef7eda3a7a1e43c48c01
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433265
Reviewed-by: Lasse Nielsen <[email protected]>
Commit-Queue: Lasse Nielsen <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/flow_analysis/flow_analysis.dart

@@ -3206,11 +3206,9 @@ class FlowModel<Type extends Object> {
       // Do not promote to `Never` (even if it would be sound to do so); it's
       // not useful.
       typeIfFalse = null;
-      if (helper.typeAnalyzerOptions.soundFlowAnalysisEnabled) {
-        ifFalseIsUnreachable = true;
-      } else {
-        // The code path might be reachable due to mixed mode unsoundness.
-      }
+      // If not sound, it might still be reachable.
+      ifFalseIsUnreachable =
+          helper.typeAnalyzerOptions.soundFlowAnalysisEnabled;
     } else if (!helper.isValidPromotionStep(
       previousType: previousType,
       newType: factoredType,
@@ -7345,9 +7343,8 @@ class _FlowAnalysisImpl<
     }
   }
 
-  /// Determines whether an expression having the given [staticType] is
-  /// guaranteed to fail an `is` or `as` check using [checkedType] due to sound
-  /// null safety.
+  /// Whether an expression having the given [staticType] is guaranteed to fail
+  /// an `is` or `as` check using [checkedType] due to sound null safety.
   ///
   /// If [TypeAnalyzerOptions.soundFlowAnalysisEnabled] is `false`, this method
   /// will return `false` regardless of its input. This reflects the fact that
@@ -7358,8 +7355,8 @@ class _FlowAnalysisImpl<
     required Type staticType,
     required Type checkedType,
   }) {
-    if (!typeAnalyzerOptions.soundFlowAnalysisEnabled) return false;
-    return typeOperations.isSubtypeOf(staticType, checkedType);
+    return typeAnalyzerOptions.soundFlowAnalysisEnabled &&
+        typeOperations.isSubtypeOf(staticType, checkedType);
   }
 
   FlowModel<Type> _join(FlowModel<Type>? first, FlowModel<Type>? second) =>

tests/language/sound_flow_analysis/full_demotion_disabled_test.dart

@@ -10,7 +10,7 @@
 import '../static_type_helper.dart';
 
 // If an assignment fully demotes a variable, types of interest are cleared.
-testFullDemotion(Object x, num n) {
+void testFullDemotion(Object x, num n) {
   x as num;
   x.expectStaticType<Exactly<num>>();
   x = '';

tests/language/sound_flow_analysis/full_demotion_test.dart

@@ -10,7 +10,7 @@
 import '../static_type_helper.dart';
 
 // If an assignment fully demotes a variable, types of interest are not cleared.
-testFullDemotion(Object x, num n) {
+void testFullDemotion(Object x, num n) {
   x as num;
   x.expectStaticType<Exactly<num>>();
   x = '';

tests/language/sound_flow_analysis/trivially_satisfied_type_check_disabled_test.dart

@@ -11,7 +11,7 @@ import '../static_type_helper.dart';
 
 // If `x` is of type `T`, flow analysis does not consider `x is T` to be
 // guaranteed to evaluate to `true`.
-testIsExact({required int intValue, required int Function() intFunction}) {
+void testIsExact({required int intValue, required int Function() intFunction}) {
   {
     // <var> is int
     int? shouldBeDemoted1 = 0;
@@ -46,7 +46,10 @@ testIsExact({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, flow analysis does not consider `x is! T` to be
 // guaranteed to evaluate to `false`.
-testIsNotExact({required int intValue, required int Function() intFunction}) {
+void testIsNotExact({
+  required int intValue,
+  required int Function() intFunction,
+}) {
   {
     // <var> is! int
     int? shouldBeDemoted1 = 0;
@@ -81,7 +84,10 @@ testIsNotExact({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, and `T <: U`, flow analysis does not consider `x is U`
 // to be guaranteed to evaluate to `true`.
-testIsSupertype({required int intValue, required int Function() intFunction}) {
+void testIsSupertype({
+  required int intValue,
+  required int Function() intFunction,
+}) {
   {
     // <var> is num
     int? shouldBeDemoted1 = 0;
@@ -116,7 +122,7 @@ testIsSupertype({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, and `T <: U`, flow analysis does not consider `x is!
 // U` to be guaranteed to evaluate to `false`.
-testIsNotSupertype({
+void testIsNotSupertype({
   required int intValue,
   required int Function() intFunction,
 }) {

tests/language/sound_flow_analysis/trivially_satisfied_type_check_test.dart

@@ -11,7 +11,7 @@ import '../static_type_helper.dart';
 
 // If `x` is of type `T`, flow analysis considers `x is T` to be guaranteed to
 // evaluate to `true`.
-testIsExact({required int intValue, required int Function() intFunction}) {
+void testIsExact({required int intValue, required int Function() intFunction}) {
   {
     // <var> is int
     int? shouldBePromoted;
@@ -46,7 +46,10 @@ testIsExact({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, flow analysis considers `x is! T` to be guaranteed
 // to evaluate to `false`.
-testIsNotExact({required int intValue, required int Function() intFunction}) {
+void testIsNotExact({
+  required int intValue,
+  required int Function() intFunction,
+}) {
   {
     // <var> is! int
     int? shouldNotBeDemoted = 0;
@@ -81,7 +84,10 @@ testIsNotExact({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, and `T <: U`, flow analysis considers `x is U` to be
 // guaranteed to evaluate to `true`.
-testIsSupertype({required int intValue, required int Function() intFunction}) {
+void testIsSupertype({
+  required int intValue,
+  required int Function() intFunction,
+}) {
   {
     // <var> is num
     int? shouldBePromoted;
@@ -116,7 +122,7 @@ testIsSupertype({required int intValue, required int Function() intFunction}) {
 
 // If `x` is of type `T`, and `T <: U`, flow analysis considers `x is! U` to be
 // guaranteed to evaluate to `false`.
-testIsNotSupertype({
+void testIsNotSupertype({
   required int intValue,
   required int Function() intFunction,
 }) {