When templates are stored in external files, developers often embed
comments to explain&|disable code.

  <!-- Oblique reference to project code name here -->
  {{if .C}}...{{else}}<!-- commented out default -->{{end}}

This unnecessarily increases the size of shipped HTML and can leak
information.

This change elides all comments of the following types:
1. <!-- ... --> comments found in source.
2. /*...*/ and // comments found in <script> elements.
3. /*...*/ and // comments found in <style> elements.

It does not elide /*...*/ or // comments found in HTML attributes:
4. <button onclick="/*...*/">
5. <div style="/*...*/">

I can find no examples of comments in attributes in Closure Templates
code and doing so would require keeping track of character positions
post decode in

  <button onclick="/&#42;...*/">

To prevent token joining, /*comments*/ are JS and CSS comments are
replaced with a whitespace char.
HTML comments are not, but to prevent token joining we could try to
detect cases like
   <<!---->b>
   </<!---->b>
which has a well defined meaning in HTML but will cause a validator
to barf.  This is difficult, and this is a very minor case.
I have punted for now, but if we need to address this case, the best
way would be to normalize '<' in stateText to '&lt;' consistently.

The whitespace to replace a JS /*comment*/ with depends on whether
there is an embedded line terminator since
    break/*
    */foo
    ...
is equivalent to
    break;
    foo
    ...
while
    break/**/foo
    ...
is equivalent to
    break foo;
    ...

Comment eliding can interfere with IE conditional comments.
http://en.wikipedia.org/wiki/Conditional_comment

<!--[if IE 6]>
<p>You are using Internet Explorer 6.</p>
<![endif]-->

/*@cc_on
  document.write("You are using IE4 or higher");
@*/

I have not encountered these in production template code, and
the typed content change in CL 4962067 provides an escape-hatch
if conditional comments are needed.

R=nigeltao
CC=golang-dev
https://golang.org/cl/4999042