Makes the references to inner JS class values explicit.

Roughly, for every inner JS class of the form:

class Outer {
  class Inner extends ParentJSClass
}

this phase creates a field Inner$jsclass in Outer to hold the JS class value for Inner. The rhs of that field is a call to a magic method, used to retain information that the back-end will need.

class Outer {
  <synthetic> val Inner$jsclass: AnyRef =
    createJSClass(classOf[Inner], js.constructorOf[ParentJSClass])

  class Inner extends ParentJSClass
}

These fields will be read by code generated in ExplicitLocalJS.

Note that this field must also be added to outer classes and traits coming from separate compilation, therefore this phase is an InfoTransform. Since the transformInfo also applies to classes defined in the current compilation unit, the tree traversal must not create the field symbols a second time when synthesizing the ValDef. Instead, it must reuse the same symbols that transformInfo will create.

It seems the easiest way to do that is to run the entire transform "in the future", with exitingPhase(ExplicitInnerJS). This design is similar to how explicitouter works.