introduce unsafe_proto

- `Array.prototype.slice` => `[].slice`

closes #1491

lib/compress.js

@@ -54,6 +54,7 @@ function Compressor(options, false_by_default) {
         drop_debugger : !false_by_default,
         unsafe        : false,
         unsafe_comps  : false,
+        unsafe_proto  : false,
         conditionals  : !false_by_default,
         comparisons   : !false_by_default,
         evaluate      : !false_by_default,
@@ -3148,6 +3149,28 @@ merge(Compressor.prototype, {
                 })
             }).optimize(compressor);
         }
+        if (compressor.option("unsafe_proto")
+            && self.expression instanceof AST_Dot
+            && self.expression.property == "prototype") {
+            var exp = self.expression.expression;
+            if (exp instanceof AST_SymbolRef && exp.undeclared()) switch (exp.name) {
+              case "Array":
+                self.expression = make_node(AST_Array, self.expression, {
+                    elements: []
+                });
+                break;
+              case "Object":
+                self.expression = make_node(AST_Object, self.expression, {
+                    properties: []
+                });
+                break;
+              case "String":
+                self.expression = make_node(AST_String, self.expression, {
+                    value: ""
+                });
+                break;
+            }
+        }
         return self.evaluate(compressor)[0];
     });