Improve error message for undeclared class property initialization

[samwgoldman]

Flow requires that class properties be declared before they can be initialized in a constructor. This behavior is reasonable, but the error message should guide the user to the solution:

/* @flow */

class Foo {
  constructor() {
    this.bar = "baz";
  }
}

test.js|5 col 5 error|  assignment of property bar
|| Property not found in
test.js|3 col 7 error|  Foo
|| 

A better error message would direct the user to declare the type of bar, as follows:

/* @flow */

class Foo {
  bar: string;
  constructor() {
    this.bar = "baz";
  }
}

|| No errors!

A better message might be "Property must be declared in class."