Change StringGetOwnProperty to produce the same strings that the lexer produces (#1460)

boa/src/builtins/string/tests.rs

@@ -1132,7 +1132,7 @@ fn string_get_property() {
     assert_eq!(forward(&mut context, "'abc'[2]"), "\"c\"");
     assert_eq!(forward(&mut context, "'abc'[3]"), "undefined");
     assert_eq!(forward(&mut context, "'abc'['foo']"), "undefined");
-    assert_eq!(forward(&mut context, "'😀'[0]"), "\"\\ud83d\"");
+    assert_eq!(forward(&mut context, "'😀'[0]"), "\"<EFBFBD>\"");
 }
 
 #[test]

boa/src/object/internal_methods.rs

@@ -620,10 +620,10 @@ impl GcObject {
                     return None;
                 }
 
-                let result_str = string.encode_utf16().nth(pos).map(|utf16_val| {
-                    char::from_u32(u32::from(utf16_val))
-                        .map_or_else(|| Value::from(format!("\\u{:x}", utf16_val)), Value::from)
-                })?;
+                let result_str = string
+                    .encode_utf16()
+                    .nth(pos)
+                    .map(|utf16_val| Value::from(String::from_utf16_lossy(&[utf16_val])))?;
 
                 let desc = PropertyDescriptor::builder()
                     .value(result_str)