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’;
}
}
```
fixes#11 (constructor and typed field semantics)
fixes#42 (Should we infer class property types from ctor args ?)
fixes#17 (number (js) should map to num (dart))
Closes#45
