[cfe][InternalNodes] Add more internal nodes

This adds internal nodes for VariableStatement, VariableDeclaration, Let, and ForStatement. The prepares for separating Variable from InternalVariable and allows for the removal of Variable.asVariableDeclaration.

Change-Id: Ib0593f64b3d22ce99c187ab4af32c8dc8cbea6e9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/509980
Reviewed-by: Chloe Stefantsova <cstefantsova@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

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

@@ -1482,8 +1482,6 @@ class VariableStatement extends Statement {
       ..parent = this;
   }
 
-  /// Returns a possibly synthesized name for this variable, consistent with
-  /// the names used across all [toString] calls.
   @override
   String toString() {
     return "VariableStatement(${toStringInternal()})";

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

@@ -71,7 +71,6 @@ abstract interface class IVariable implements TreeNode, Annotatable {
   bool get hasIsWildcard;
   bool get hasIsSuperInitializingFormal;
   bool get hasIsErroneouslyInitialized;
-  Variable get asVariableDeclaration;
 }
 
 /// The root of the sealed hierarchy of non-type variables.
@@ -186,9 +185,6 @@ sealed class Variable extends VariableBase
   @override
   bool get isAssignable;
 
-  @override
-  Variable get asVariableDeclaration => this;
-
   abstract String? name;
 
   @override
@@ -646,9 +642,6 @@ class LegacyVariable extends TreeNode implements Variable, Annotatable {
     throw new UnsupportedError("${this.runtimeType}.context=");
   }
 
-  @override
-  Variable get asVariableDeclaration => this;
-
   @override
   Variable get variable => this;