Polymer: missing <script>/noscript in custom element definition not diagnosed

[DartBot]

This issue was originally filed by serg...@chromium.org

---

A polymer-element definition is supposed to provide either a <script> tag or a 'noscript' attribute. However, when both are missing, no useful diagnostic is output to the console, while the element is broken (doesn't successfully instantiate/render). There should be an error message for this.

[madsager]

Added Area-Polymer, Triaged labels.

[jmesserly]

A polymer-element definition is supposed to provide either a <script> tag or a 'noscript' attribute.

fyi, Polymer (JS|Dart) supports associating the script after the fact. This is an important scenario because it allows a single script to supply the definitions for multiple custom elements

---

Added Duplicate label.
Marked as being merged into #14054.

[DartBot]

This comment was originally written by serg...@chromium.org

---

Could this be explicitly indicated by some tag, e.g. 'deferred-script'? This way, if none of 'noscript', 'deferred-script' or <script> was found, an error would be reported.