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’;
  }
}
```

modules/test_lib/test/test_lib_spec.js

@@ -2,6 +2,7 @@ import {describe, it, iit, ddescribe, expect} from 'test_lib/test_lib';
 import {MapWrapper} from 'facade/collection';
 
 class TestObj {
+  prop;
   constructor(prop) {
     this.prop = prop;
   }