[kernel][Contexts] Add VariableDeclaration

This adds a VariableDeclaration node which is used in ForStatement instead of VariableStatement.

This is a step towards removing the initializer from Variable. Long term, VariableDeclaration will own the initializer expression for variables and function parameters will have a defaultValue property instead of using the initializer property for the default value.

TEST=existing

Change-Id: I4a663eeb6006a0f9f098fb2b3e3b502d2ae583b0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/505681
Reviewed-by: Stephen Adams <sra@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>

pkg/dart2bytecode/lib/bytecode_generator.dart

@@ -4888,13 +4888,13 @@ class BytecodeGenerator extends RecursiveVisitor {
   }
 
   @override
-  void visitLegacyVariableStatement(LegacyVariableStatement node) {
+  void visitVariableDeclaration(VariableDeclaration node) {
     _handleVariableInitialization(node.variable);
   }
 
   @override
-  void visitVariableInitialization(VariableInitialization node) {
-    _handleVariableInitialization(node.variable);
+  void visitVariableStatement(VariableStatement node) {
+    visitVariableDeclaration(node.declaration);
   }
 
   void _handleVariableInitialization(Variable node) {