[cfe] Add FunctionNode.thisVariable

In the new variable model `this` is treate as a variable. Similarly to
other variables, it is placed into a variable context, which is
treated as the point of declaration of `this` variable. Since
parameter lists of a `FunctionNode`, such as
`FunctionNode.positionalParameters` and
`FunctionNode.namedParameters`, aren't treated as declarations either,
but rather as a convenient way to access the parameters, a similar
treatment of `this` variable is introduced in this CL, where it
becomes accessible via `FunctionNode.thisVariable`, in addition to
being declared in an appropriate variable context.

Part of https://github.com/dart-lang/sdk/issues/61572

Change-Id: Ia608fd01661353eb704de225f6b123900b1671b8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/494840
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Commit-Queue: Chloe Stefantsova <cstefantsova@google.com>

pkg/kernel/lib/clone.dart

@@ -886,6 +886,7 @@ class CloneVisitorNotMembers
         .map(clone)
         .toList();
     List<VariableDeclaration> named = node.namedParameters.map(clone).toList();
+    VariableDeclaration? thisVariable = cloneOptional(node.thisVariable);
     final DartType? futureValueType = node.emittedValueType != null
         ? visitType(node.emittedValueType!)
         : null;
@@ -894,6 +895,7 @@ class CloneVisitorNotMembers
       typeParameters: typeParameters,
       positionalParameters: positional,
       namedParameters: named,
+      thisVariable: thisVariable,
       requiredParameterCount: node.requiredParameterCount,
       returnType: visitType(node.returnType),
       asyncMarker: node.asyncMarker,

pkg/kernel/lib/src/ast/functions.dart

@@ -41,6 +41,7 @@ class FunctionNode extends TreeNode implements ScopeProvider, ContextConsumer {
   int requiredParameterCount;
   List<VariableDeclaration> positionalParameters;
   List<VariableDeclaration> namedParameters;
+  VariableDeclaration? thisVariable;
   DartType returnType; // Not null.
   Statement? _body;
 
@@ -110,6 +111,7 @@ class FunctionNode extends TreeNode implements ScopeProvider, ContextConsumer {
     this.asyncMarker = AsyncMarker.Sync,
     AsyncMarker? dartAsyncMarker,
     this.emittedValueType,
+    this.thisVariable,
   }) : this.positionalParameters =
            positionalParameters ?? <VariableDeclaration>[],
        this.requiredParameterCount =

pkg/kernel/lib/src/equivalence.dart

@@ -4367,6 +4367,9 @@ class EquivalenceStrategy {
     if (!checkFunctionNode_namedParameters(visitor, node, other)) {
       result = visitor.resultOnInequivalence;
     }
+    if (!checkFunctionNode_thisVariable(visitor, node, other)) {
+      result = visitor.resultOnInequivalence;
+    }
     if (!checkFunctionNode_returnType(visitor, node, other)) {
       result = visitor.resultOnInequivalence;
     }
@@ -10832,6 +10835,18 @@ class EquivalenceStrategy {
     );
   }
 
+  bool checkFunctionNode_thisVariable(
+    EquivalenceVisitor visitor,
+    FunctionNode node,
+    FunctionNode other,
+  ) {
+    return visitor.checkNodes(
+      node.thisVariable,
+      other.thisVariable,
+      'thisVariable',
+    );
+  }
+
   bool checkFunctionNode_returnType(
     EquivalenceVisitor visitor,
     FunctionNode node,