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/examples/src/hello_world/app.js

@@ -29,6 +29,7 @@ import {bootstrap, Component, Decorator, TemplateConfig, NgElement} from 'core/c
   })
 })
 class HelloCmp {
+  greeting: string;
   constructor(service: GreetingService) {
     this.greeting = service.greeting;
   }
@@ -49,6 +50,7 @@ class RedDec {
 
 // A service used by the HelloCmp component.
 class GreetingService {
+  greeting:string;
   constructor() {
     this.greeting = 'hello';
   }