[3.0] Consider renaming polymer.js

[justinfagnani]

The file /polymer.html was named so for backwards compatibility with Polymer 1.0, so that hybrid elements could use the same import. We don't have this constraint with 3.0, and could rename polymer.js to polymer-legacy.js, which would allow us to create a new polymer.js with only modern exports.

[joeblew99]

Why break everyone that need to stick with polymer 1 - 2 for a fair while ?
Instead use a new name for polymer 3.0. maybe polymer 3.js

[arthurevans]

This change--if we make it--wouldn't affect 1.x or 2.x at all--they import an html file, not a js file. You'll need to change how you import polymer when you go from 2 to 3 whether or not we make this change.

[joeblew99]

ah i missed the .html and .js. Makes sense now. thanks @arthurevans

[baocang]

Move backwards compatibility version to -legacy is better. I like always keep polymer.html|js as modern.

[justinfagnani]

If we choose to do this, we can do it one of two ways: Hard-code a filename remapping in polymer-modulizer, or add the ability to make a directive in a file to have modulizer rename it:

ie, in polymer-element.html:

<!-- modulizer-rename: polymer.js -->

[bicknellr]

(Blocked on Polymer/polymer-modulizer#354.)

[bicknellr]

#367 is merged.