enhance global_defs

- support arrays, objects & AST_Node
- support `"a.b":1` on both cli & API
- emit warning if variable is modified
- override top-level variables

fixes #1416
closes #1198
closes #1469

lib/scope.js

@@ -206,14 +206,7 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
                 node.scope.uses_arguments = true;
             }
             if (!sym) {
-                if (globals.has(name)) {
-                    sym = globals.get(name);
-                } else {
-                    sym = new SymbolDef(self, globals.size(), node);
-                    sym.undeclared = true;
-                    sym.global = true;
-                    globals.set(name, sym);
-                }
+                sym = self.def_global(node);
             }
             node.thedef = sym;
             node.reference(options);
@@ -227,6 +220,19 @@ AST_Toplevel.DEFMETHOD("figure_out_scope", function(options){
     }
 });
 
+AST_Toplevel.DEFMETHOD("def_global", function(node){
+    var globals = this.globals, name = node.name;
+    if (globals.has(name)) {
+        return globals.get(name);
+    } else {
+        var g = new SymbolDef(this, globals.size(), node);
+        g.undeclared = true;
+        g.global = true;
+        globals.set(name, g);
+        return g;
+    }
+});
+
 AST_Scope.DEFMETHOD("init_scope_vars", function(){
     this.variables = new Dictionary(); // map name to AST_SymbolVar (variables defined in this scope; includes functions)
     this.functions = new Dictionary(); // map name to AST_SymbolDefun (functions defined in this scope)