compiler: add Stmt.isOnceFieldInit

src/compiler/fan/ast/Stmt.fan

@@ -38,6 +38,12 @@ abstract class Stmt : Node
   **
   abstract Bool isDefiniteAssign(|Expr lhs->Bool| f)
 
+  **
+  ** Is this statement a field initialization for a once storage field.
+  ** This is used in compilerEs to skip initializing fields to "_once_"
+  **
+  virtual Bool isOnceFieldInit() { false }
+
 //////////////////////////////////////////////////////////////////////////
 // Tree
 //////////////////////////////////////////////////////////////////////////
@@ -112,6 +118,14 @@ class ExprStmt : Stmt
 
   override Bool isDefiniteAssign(|Expr lhs->Bool| f) { expr.isDefiniteAssign(f) }
 
+  override Bool isOnceFieldInit()
+  {
+    if (expr.id !== ExprId.assign) return false
+    x := ((BinaryExpr)expr).lhs as FieldExpr
+    if (x === null) return false
+    return x.field.isOnce
+  }
+
   override Void walkChildren(Visitor v, VisitDepth depth)
   {
     expr = walkExpr(v, depth, expr)
@@ -657,4 +671,5 @@ enum class StmtId
   continueStmt,
   tryStmt,
   switchStmt
-}
+}
+