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Issue #18037 has been merged into this issue.

Pasting here issue #7 to track it all in one place:

A lot of our transformers are generic about web-components features. For example:

• import-inliner applies to anything that uses html imports
• @­CustomTag should apply to any custom tag, not just polymer tags

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To make this extensible, one though is to include in the annotation the logic for how is the registration supposed to be done. For example, consider we rename @­CustomTag to @­RegisterElement, which has the following code:

class RegisterElement {
  final String tagName;
  const RegisterElement(this.tagName);

  register(Type t) => document.registerElement(tagName, t);

}

A generic transformer can detect that if you have:

  @­RegisterElement('x-foo')
  class Foo ... {}

it can copy the annotation as a const expression and generate a registration call by invoking a method on that const object:

  const RegisterElement('x-foo').register(Foo);

With this approach, it's easy to later add new kinds of registration calls. For example, Polymer can define a subclass of this annotation:

  class RegisterPolymerElement extends RegisterElement {

    @­override
    register(Type t) => Polymer.register(tagName, t);
  }

So the generic transformer can detect that if you have:

  @­RegisterPolymerElement('x-foo')
  class Foo ... {}

it will correctly generate the following call:

  const RegisterPolymerElement('x-foo').register(Foo);

This issue has been moved to dart-archive/polymer-dart#383.