Keep the receiver read in every JavaScript function for now

Skipping it in functions without `this` missed methods that read the
receiver through super (8302-class-super-method-values,
prototype-method-overrides) and changes the default build's dynamic-tier
counts. It changes the default JavaScript lowering, so it gets its own PR.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/compiler/src/frontend/lowering/lower-functions.ts

@@ -62,29 +62,6 @@ function declareCallReceiver(
   };
 }
 
-/** Whether a JavaScript function's own `this` can be read: a `this` in its
- * parameters or body, outside nested non-arrow functions (which bind their
- * own). Conservative inside classes and object literal methods. A function
- * that never reads it skips the ambient receiver read at entry. */
-function readsOwnThis(node: ts.FunctionLikeDeclaration): boolean {
-  let found = false;
-  const visit = (n: ts.Node): void => {
-    if (found) return;
-    if (n.kind === ts.SyntaxKind.ThisKeyword) {
-      found = true;
-      return;
-    }
-    if (ts.isFunctionDeclaration(n) || ts.isFunctionExpression(n)) return;
-    ts.forEachChild(n, (c) => {
-      visit(c);
-      return undefined;
-    });
-  };
-  for (const p of node.parameters.nodes) visit(p);
-  if (node.body !== undefined) visit(node.body);
-  return found;
-}
-
 /** True when the identifier resolves (through import aliases) to a
  * top-level function declaration of ANY program file (not merely a
  * same-named local shadowing one). Functions declared directly in a
@@ -359,7 +336,7 @@ export function lowerLambda(
   try {
     const receiver =
       (checkedReceiver ||
-        (isJsSourceFile(ts.getSourceFileOfNode(node)) && readsOwnThis(node)) ||
+        isJsSourceFile(ts.getSourceFileOfNode(node)) ||
         node.parameters.nodes.some(isThisParameter)) &&
       !ts.isArrowFunction(node)
         ? declareCallReceiver(lowerer, node, isGenerator)
@@ -564,8 +541,7 @@ export function lowerFunction(lowerer: Lowerer, decl: ts.FunctionDeclaration): I
     const argumentCount: IrStmt[] = [];
     declareArgumentCount(lowerer, decl, argumentCount);
     const receiver =
-      decl.parameters.nodes.some(isThisParameter) ||
-      (isJsSourceFile(ts.getSourceFileOfNode(decl)) && readsOwnThis(decl))
+      decl.parameters.nodes.some(isThisParameter) || isJsSourceFile(ts.getSourceFileOfNode(decl))
         ? declareCallReceiver(lowerer, decl, sig.generator !== undefined)
         : null;
     const { params, prologue } = lowerer.declareParams(decl.parameters.nodes, sig.params);