Support property observers which are direct function references

[ChadKillingsworth]

Partially addresses #4543

Provides better static analysis and refactoring support in multiple tools. Alleviates the need for property reflection with Closure-compiler renaming.

The test case included isn't actually that useful (but does test the change). Real benefits come with the class style syntax:

class FooElement extends Polymer.Element {
  static get is() { return 'foo-element'; }
  static get properties() {
    return {
      bar: {
        type: String,
        observer: FooElement.prototype.barChanged_
      }
    }
  }
  barChanged_(newValue, oldValue) {
    console.log(newValue, oldValue)
  }
}

[TimvdLippe]

This SFTM. Could you resolve the merge conflicts and then @kevinpschaaf can probably take a look?

[ChadKillingsworth]

Sorry for the delay. Rebased to tip of tree.

[TimvdLippe]

Talked about this PR with @kevinpschaaf In general we feel this is a small quality of life improvement by using the language semantics. We were initially reluctant, as this PR introduce a new way for the same thing. However, as the benefits are substantial and for this case it is small, we are okay with the change.

We need to update the docs for this change as well @arthurevans

[aomarks]

Filed Polymer/old-docs-site#2421 to add documentation.

[aomarks]

Filed Polymer/old-docs-site#2421 to add documentation.

Oops, ignore me, missed that there already was an issue.