avm2: Re-implement RegExp.prototype.toString

Also remove the TObject to_string implementation that isn't necessary.

core/src/avm2/globals/RegExp.as

@@ -29,9 +29,34 @@ package {
         }
 
         prototype.toString = function():String {
-            return this.valueOf();
+            // Note: This function is not generic and will throw for non-regexps.
+            var regexp: RegExp = this;
+
+            // ECMA-262 Edition 5.1 - RegExp.prototype.toString():
+            //  Return the String value formed by concatenating the Strings "/",
+            //  the String value of the source property of this RegExp object, and "/";
+            //  plus "g" if the global property is true,
+            //  "i" if the ignoreCase property is true,
+            //  and "m" if the multiline property is true.
+            var string = "/" + regexp.source + "/";
+            if (regexp.global) {
+                string += "g";
+            }
+            if (regexp.ignoreCase) {
+                string += "i";
+            }
+            if (regexp.multiline) {
+                string += "m";
+            }
+            if (regexp.dotall) {
+                string += "s";
+            }
+            if (regexp.extended) {
+                string += "x";
+            }
+            return string;
         }
-        
+
         prototype.setPropertyIsEnumerable("exec", false);
         prototype.setPropertyIsEnumerable("test", false);
         prototype.setPropertyIsEnumerable("toString", false);

core/src/avm2/object/regexp_object.rs

@@ -87,10 +87,6 @@ impl<'gc> TObject<'gc> for RegExpObject<'gc> {
         self.0.as_ptr() as *const ObjectPtr
     }
 
-    fn to_string(&self, _activation: &mut Activation<'_, 'gc>) -> Result<Value<'gc>, Error<'gc>> {
-        Ok(Value::Object(Object::from(*self)))
-    }
-
     fn value_of(&self, mc: &Mutation<'gc>) -> Result<Value<'gc>, Error<'gc>> {
         let read = self.0.read();
         let mut s = WString::new();