Support auto-accessors

[duncanmak]

Version: Deno 1.40.2

I'm trying to use Deno as my complete toolchain for writing a component using Lit. I'm trying out the new Stage 3 decorators.

When I compile using "experimentalDecorators": false, the accessor properties stay in the generated code, instead of being down-leveled.

For code like this,

@property()
accessor name = "World";

@property({ type: Number })
accessor count = 0;

tsc can generate output like this:

    get name() {
      return __classPrivateFieldGet(this, _MyElement_name_accessor_storage, "f");
    }
    set name(value) {
      __classPrivateFieldSet(this, _MyElement_name_accessor_storage, value, "f");
    }
    get count() {
      return __classPrivateFieldGet(this, _MyElement_count_accessor_storage, "f");
    }
    set count(value) {
      __classPrivateFieldSet(this, _MyElement_count_accessor_storage, value, "f");
    }

I'm using this tsconfig.json in tsc 5.3.3:

"compilerOptions": {
    "target": "ES6",
    "experimentalDecorators": false,
    "useDefineForClassFields": true,
    "forceConsistentCasingInFileNames": true,
    "strict": true,
    "module": "esnext",
    "moduleResolution": "Bundler",
    "esModuleInterop": true,
    "verbatimModuleSyntax": true,
    "sourceMap": true,
  },