[dart2js] Use null-aware elements

Change-Id: I2dc74c57efd355ab4871eb2ad48a97fc817a2940
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/433300
Reviewed-by: Mayank Patke <fishythefish@google.com>
Commit-Queue: Stephen Adams <sra@google.com>

pkg/compiler/analysis_options.yaml

@@ -21,3 +21,4 @@ linter:
     - no_default_cases
     - unnecessary_ignore
     - unnecessary_statements
+    - use_null_aware_elements

pkg/compiler/lib/src/elements/types.dart

@@ -1532,8 +1532,7 @@ class _DartTypeToStringVisitor extends DartTypeVisitor<void, void> {
       // Assign names to _DeferredNames that were not assigned while visiting a
       // generic function type.
       Set<String> usedNames = {
-        for (final deferred in variableToName.values)
-          if (deferred.name != null) deferred.name!,
+        for (final deferred in variableToName.values) ?deferred.name,
       };
       int startGroup = (_genericFunctions?.length ?? 0) + 1;
       for (var entry in variableToName.entries) {

pkg/compiler/lib/src/js/rewrite_async.dart

@@ -2558,8 +2558,7 @@ class AsyncStarRewriter extends AsyncRewriterBase {
     List<int> enclosingFinallyLabels = [
       // At the bottom of the stack is the return label.
       exitLabel!,
-      for (final node in jumpTargets)
-        if (finallyLabels[node] != null) finallyLabels[node]!,
+      for (final node in jumpTargets) ?finallyLabels[node],
     ];
 
     addStatement(

pkg/compiler/lib/src/js_model/js_to_frontend_map.dart

@@ -101,7 +101,7 @@ class JsToFrontendMap {
     return {
       for (final member in set.map(toBackendMember))
         // Members that are not live don't have a corresponding backend member.
-        if (member != null) member,
+        ?member,
     };
   }
 

pkg/compiler/lib/src/kernel/transformations/modular/list_factory_specializer.dart

@@ -170,7 +170,7 @@ class ListFactorySpecializer extends BaseSpecializer {
     )..fileOffset = node.fileOffset;
 
     return BlockExpression(
-      Block([if (lengthVariable != null) lengthVariable!, listVariable, loop]),
+      Block([?lengthVariable, listVariable, loop]),
       VariableGet(listVariable)..fileOffset = node.fileOffset,
     );
   }

pkg/compiler/lib/src/ssa/nodes.dart

@@ -3978,7 +3978,7 @@ abstract class HLateCheck extends HCheck {
     HInstruction? name,
     this.isTrusted,
     AbstractValue type,
-  ) : super([input, if (name != null) name], type);
+  ) : super([input, ?name], type);
 
   bool get hasName => inputs.length > 1;
 
@@ -4989,13 +4989,7 @@ class HArrayFlagsCheck extends HCheck {
     HInstruction? operation,
     HInstruction? verb,
     AbstractValue type,
-  ) : super([
-        array,
-        arrayFlags,
-        checkFlags,
-        if (operation != null) operation,
-        if (verb != null) verb,
-      ], type);
+  ) : super([array, arrayFlags, checkFlags, ?operation, ?verb], type);
 
   HInstruction get array => inputs[0];
   HInstruction get arrayFlags => inputs[1];