[strong] make function and class declarations lexical & immutable

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 42c8177251e89824f04003e0905703f1c34fac9c..faccb90457675627e553b42291f40c75b26b9dce 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -577,8 +577,7 @@ class FunctionDeclaration FINAL : public Declaration {
                       int pos)
       : Declaration(zone, proxy, mode, scope, pos),
         fun_(fun) {
-    // At the moment there are no "const functions" in JavaScript...
-    DCHECK(mode == VAR || mode == LET);
+    DCHECK(mode == VAR || mode == LET || mode == CONST);
     DCHECK(fun != NULL);
   }