[cfe][Contexts] Flatten variables hierarchy

This removes the LegacyVariableDeclaration and VariableInitializationBase classes to create a simpler hierarchy. This is done preparation for splitting variables from statements.

TEST=existing

Change-Id: If29c9eee1e3d8bed819ce53ed178c3efa3bffaea
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/501961
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>

pkg/dart2bytecode/lib/bytecode_generator.dart

@@ -4860,10 +4860,7 @@ class BytecodeGenerator extends RecursiveVisitor {
     finallyBlocks.remove(node);
   }
 
-  bool _skipVariableInitialization(
-    VariableInitializationBase v,
-    bool isCaptured,
-  ) {
+  bool _skipVariableInitialization(VariableDeclaration v, bool isCaptured) {
     // We can skip variable initialization if the variable is supposed to be
     // initialized to null and it's captured. This is because all the slots in
     // the capture context are implicitly initialized to null.
@@ -4886,11 +4883,11 @@ class BytecodeGenerator extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableInitialization(VariableInitializationBase node) {
+  void visitVariableInitialization(VariableInitialization node) {
     _handleVariableInitialization(node);
   }
 
-  void _handleVariableInitialization(VariableInitializationBase node) {
+  void _handleVariableInitialization(VariableDeclaration node) {
     if (!node.isConst) {
       final bool isCaptured = locals.isCaptured(node.variable);
       final initializer = node.initializer;

pkg/dart2bytecode/lib/local_vars.dart

@@ -553,11 +553,11 @@ class _ScopeBuilder extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableInitialization(VariableInitializationBase node) {
+  void visitVariableInitialization(VariableInitialization node) {
     _handleVariableInitialization(node);
   }
 
-  void _handleVariableInitialization(VariableInitializationBase node) {
+  void _handleVariableInitialization(VariableDeclaration node) {
     _declareVariable(node.variable);
     node.visitChildren(this);
   }
@@ -1097,11 +1097,11 @@ class _Allocator extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableInitialization(VariableInitializationBase node) {
+  void visitVariableInitialization(VariableInitialization node) {
     _handleVariableInitialization(node);
   }
 
-  void _handleVariableInitialization(VariableInitializationBase node) {
+  void _handleVariableInitialization(VariableDeclaration node) {
     _allocateVariable(node.variable);
     node.visitChildren(this);
   }