Flow analysis: inline _removePromotedAll at the call site.

There was only one call site, and the logic is clearer if all the
logic is in one method.

Change-Id: I59a6e0ce1e3345039146525a9bcd0b72025cbb5d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/115006
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/front_end/lib/src/fasta/flow_analysis/flow_analysis.dart

@@ -884,11 +884,19 @@ class FlowModel<Variable, Type> {
   /// be able to remove this method.
   FlowModel<Variable, Type> removePromotedAll(
       Iterable<Variable> variables, Set<Variable> referencedVariables) {
-    Map<Variable, VariableModel<Type>> newVariableInfo =
-        _removePromotedAll(variableInfo, variables, referencedVariables);
-
-    if (identical(newVariableInfo, variableInfo)) return this;
-
+    Map<Variable, VariableModel<Type>> newVariableInfo;
+    for (Variable variable in variables) {
+      assert(() {
+        referencedVariables?.add(variable);
+        return true;
+      }());
+      VariableModel<Type> info = variableInfo[variable];
+      if (info?.promotedType != null) {
+        (newVariableInfo ??= new Map<Variable, VariableModel<Type>>.from(
+            variableInfo))[variable] = info.withPromotedType(null);
+      }
+    }
+    if (newVariableInfo == null) return this;
     return new FlowModel<Variable, Type>._(reachable, newVariableInfo);
   }
 
@@ -971,34 +979,6 @@ class FlowModel<Variable, Type> {
     return _updateVariableInfo(variable, newInfoForVar);
   }
 
-  /// Updates a "variableInfo" [map] to indicate that a set of [variable] is no
-  /// longer promoted, treating the map as immutable.
-  ///
-  /// If assertions are enabled and [referencedVariables] is not `null`, all
-  /// variables in [variables] will be stored in [referencedVariables] as a side
-  /// effect of this call.
-  Map<Variable, VariableModel<Type>> _removePromotedAll(
-      Map<Variable, VariableModel<Type>> map,
-      Iterable<Variable> variables,
-      Set<Variable> referencedVariables) {
-    if (map.isEmpty) return const {};
-    Map<Variable, VariableModel<Type>> result;
-    for (Variable variable in variables) {
-      assert(() {
-        referencedVariables?.add(variable);
-        return true;
-      }());
-      VariableModel<Type> info = map[variable];
-      if (info?.promotedType != null) {
-        (result ??=
-                new Map<Variable, VariableModel<Type>>.from(map))[variable] =
-            info.withPromotedType(null);
-      }
-    }
-    if (result == null) return map;
-    return result;
-  }
-
   /// Returns a new [FlowModel] where the information for [variable] is replaced
   /// with [model].
   FlowModel<Variable, Type> _updateVariableInfo(