chore: Make field declarations explicit

This used to be valid code:

```
class Foo {
  constructor() {
    this.bar = ‘string’;
  }
}
```

This will now fail since ‘bar’ is not explicitly
defined as a field. We now have to write:

```
class Foo {
  bar:string; // << REQUIRED
  constructor() {
    this.bar = ‘string’;
  }
}
```

tools/transpiler/spec/arrow_functions_spec.js

@@ -13,6 +13,8 @@ var max = (a, b) => {
 };
 
 class LexicalThis {
+  zero;
+
   constructor() {
     this.zero = 0;
   }