Add [] and []= operators for unnamed getters and setters

[srujzs]

Closes https://github.com/dart-lang/web/issues/127

The IDL defines unnamed getters or setters which should be emitted as special variants of the index operators.

[srujzs]

cc @rutvik110 who was curious about generator changes. In general, changes go into translator.dart, which contain the following:

• Intermediate representations when translating from the parsed IDL to the resulting Dart code.
• Code to visit the parsed IDL and create IR nodes, including interfacelikes, attributes, operations, types, and more. -- This is where the real bulk is, especially when dealing with types. -- Here, we use the special field of IDL operations to give different names to some operations.
• Code to process the resulting IR nodes and emit them as Dart code (using package:code_builder which itself is a representation of Dart code). -- Here, we just make sure that []= members don't contain an invalid return type when emitting them as Dart code.

The definition for the parsed IDL comes from https://github.com/w3c/webidl2.js.

[kevmoo]

Since we're using..."weird" operators, would it make sense to add a simple integration test...for just one of the types here.?

[srujzs]

Since we're using..."weird" operators, would it make sense to add a simple integration test...for just one of the types here.?

Yup, I tested this locally anyways, so no reason not to add a test for it. I added a smoke test for examples of [] and []=.