[cfe][Contexts] Split VariableDeclaration and VariableStatement

This separates VariableDeclaration from Statement. VariableDeclaration no longer implements Statement and variable declared in a block or in a for-statement are now wrapped by a VariableStatement.

Currently there are two VariableStatement implementations; LegacyVariableStatement for variables in the current model, called LegacyVariable, and VariableInitialization for variables used in the new, still experimental, encoding that supports scope computation.

This CL is a step towards realigning the AST nodes to the new model in which each kind of variable has its own distinct subclass. (LocalVariable, PositionalParameter, NamedParameter, SyntheticVariable, etc.)

Note that it is not the intent to use VariableStatement in ForStatement going forward but that will be handled in a follow-up.

TEST=existing.

Change-Id: I5b309cd62c9b138f95b74fb054686edffa49a393
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/502681
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/dart2bytecode/lib/bytecode_generator.dart

@@ -4832,13 +4832,18 @@ class BytecodeGenerator extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableDeclaration(VariableDeclaration node) {
+  void defaultVariableDeclaration(VariableDeclaration node) {
     _handleVariableInitialization(node);
   }
 
+  @override
+  void visitLegacyVariableStatement(LegacyVariableStatement node) {
+    _handleVariableInitialization(node.variable);
+  }
+
   @override
   void visitVariableInitialization(VariableInitialization node) {
-    _handleVariableInitialization(node);
+    _handleVariableInitialization(node.variable);
   }
 
   void _handleVariableInitialization(VariableDeclaration node) {

pkg/dart2bytecode/lib/local_vars.dart

@@ -527,41 +527,11 @@ class _ScopeBuilder extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableDeclaration(VariableDeclaration node) {
-    _handleVariableInitialization(node);
-  }
-
-  @override
-  void visitVariableInitialization(VariableInitialization node) {
-    _handleVariableInitialization(node);
-  }
-
-  void _handleVariableInitialization(VariableDeclaration node) {
+  void defaultVariableDeclaration(VariableDeclaration node) {
     _declareVariable(node.variable);
     node.visitChildren(this);
   }
 
-  @override
-  void visitPositionalParameter(PositionalParameter node) {
-    _handleFunctionParameter(node);
-  }
-
-  @override
-  void visitNamedParameter(NamedParameter node) {
-    _handleFunctionParameter(node);
-  }
-
-  void _handleFunctionParameter(FunctionParameter node) {
-    _declareVariable(node);
-    node.visitChildren(this);
-  }
-
-  @override
-  void visitCatchVariable(CatchVariable node) {
-    _declareVariable(node);
-    node.visitChildren(this);
-  }
-
   @override
   void visitVariableGet(VariableGet node) {
     _useVariable(node.variable);
@@ -1063,16 +1033,7 @@ class _Allocator extends RecursiveVisitor {
   }
 
   @override
-  void visitVariableDeclaration(VariableDeclaration node) {
-    _handleVariableInitialization(node);
-  }
-
-  @override
-  void visitVariableInitialization(VariableInitialization node) {
-    _handleVariableInitialization(node);
-  }
-
-  void _handleVariableInitialization(VariableDeclaration node) {
+  void defaultVariableDeclaration(VariableDeclaration node) {
     _allocateVariable(node.variable);
     node.visitChildren(this);
   }