Changing reserved["="] to symbol("=") for better parsing results.

Better parsing on extended attributes by switching from reserved to symbol. Might consider switching some other reserves to symbols at some point. All tests pass on `web_idl_test.dart`, this test file will get restructured at some point. 

On the `web_idl_files_test.dart` file 

Before commit

```
66 PASSED, 0 FAILED, 36 ERRORS
```

After commit

```
74 PASSED, 0 FAILED, 28 ERRORS
```

What files now pass

```
test/idl/valid/namedconstructor.widl
test/idl/valid/putforwards.widl
test/idl/valid/treatasnull.widl
test/idl/valid/treatasundefined.widl
idl/app_runtime.idl
idl/downloads_internal.idl
idl/experimental_idltest.idl
idl/mdns.idl
```

What idl files still fail

```
ERROR: Test valid web idl files Testing test/idl/valid/sequence.widl
  Test failed: Caught line 4, character 29: unexpected 'f'.
ERROR: Test valid web idl files Testing test/idl/valid/constants.widl
  Test failed: Caught line 10, character 22: expected '-', 'null', 'Infinity', float, integer, 'true', 'false' or 'NaN', got '.'.
ERROR: Test valid web idl files Testing test/idl/valid/iterator.widl
  Test failed: Caught line 5, character 19: expected '(', got ';'.
ERROR: Test valid web idl files Testing test/idl/valid/serializer.widl
  Test failed: Caught line 8, character 13: expected identifier, '?', '(', '[' or spaces, got ';'.
ERROR: Test valid web idl files Testing test/idl/valid/static.widl
  Test failed: Caught line 9, character 10: expected 'DOMString', 'byte', 'Date', 'octet', identifier, 'double', 'unrestricted', 'any', 'object', 'boolean', 'short', 'void', 'float', 'sequence', 'long', 'unsigned' or '(', got 'r'.
ERROR: Test valid web idl files Testing test/idl/valid/typedef.widl
  Test failed: Caught line 7, character 24: unexpected 'P'.  
ERROR: Test valid web idl files Testing test/idl/valid/typesuffixes.widl
  Test failed: Caught line 2, character 22: unexpected 'D'. 
ERROR: Test chrome idl files Testing test/../idl/app_window.idl
  Test failed: Caught line 17, character 16: expected string literal, got 'n'.
ERROR: Test chrome idl files Testing test/../idl/braille_display_private.idl
  Test failed: Caught line 9, character 5: expected string literal, got 'l'.
ERROR: Test chrome idl files Testing test/../idl/desktop_capture.idl
  Test failed: Caught line 11, character 5: expected string literal, got 's'.
ERROR: Test chrome idl files Testing test/../idl/developer_private.idl
  Test failed: Caught line 11, character 5: expected string literal, got 'h'.
ERROR: Test chrome idl files Testing test/../idl/dial.idl
  Test failed: Caught line 25, character 5: expected string literal, got 'n'.
ERROR: Test chrome idl files Testing test/../idl/downloads.idl
  Test failed: Caught line 7, character 14: unexpected 'd'.
ERROR: Test chrome idl files Testing test/../idl/feedback_private.idl
  Test failed: Caught line 11, character 17: unexpected 'B'.
ERROR: Test chrome idl files Testing test/../idl/file_system.idl
  Test failed: Caught line 31, character 5: expected string literal, got 'o'.
ERROR: Test chrome idl files Testing test/../idl/media_galleries.idl
  Test failed: Caught line 11, character 5: expected string literal, got 'n'.
ERROR: Test chrome idl files Testing test/../idl/notifications.idl
  Test failed: Caught line 10, character 5: expected string literal, got 'b'.
ERROR: Test chrome idl files Testing test/../idl/power.idl
  Test failed: Caught line 10, character 5: expected string literal, got 's'.
ERROR: Test chrome idl files Testing test/../idl/recovery_private.idl
  Test failed: Caught line 26, character 5: expected string literal, got 'c'.
ERROR: Test chrome idl files Testing test/../idl/rtc_private.idl
  Test failed: Caught line 8, character 20: expected whitespace/comment or string literal, got 'c'.
ERROR: Test chrome idl files Testing test/../idl/serial.idl
  Test failed: Caught line 11, character 18: expected string literal, got 's'.
ERROR: Test chrome idl files Testing test/../idl/signed_in_devices.idl
  Test failed: Caught line 9, character 5: expected string literal, got 'w'.
ERROR: Test chrome idl files Testing test/../idl/socket.idl
  Test failed: Caught line 9, character 5: expected string literal, got 't'.
ERROR: Test chrome idl files Testing test/../idl/sync_file_system.idl
  Test failed: Caught line 13, character 5: expected string literal, got 'a'.
ERROR: Test chrome idl files Testing test/../idl/system_indicator.idl
  Test failed: Caught line 11, character 8: expected whitespace/comment, identifier, '[' or spaces, got '?'.
ERROR: Test chrome idl files Testing test/../idl/system_storage.idl
  Test failed: Caught line 12, character 5: expected string literal, got 'f'.
ERROR: Test chrome idl files Testing test/../idl/tab_capture.idl
  Test failed: Caught line 10, character 5: expected string literal, got 'p'.
ERROR: Test chrome idl files Testing test/../idl/usb.idl
  Test failed: Caught line 12, character 19: expected whitespace/comment or string literal, got 'i'.
```