Add a temporary html5 externs file

This is needed because the current version of closure compiler we use does not
include externs for valueAsNumber which we now use in ol.dom.Input.
So this backports patch from issue:
https://code.google.com/p/closure-compiler/issues/detail?id=1060

This patch should be reverted when the next closure compiler release is out
with valueAsNumber externs.

build.py

@@ -316,6 +316,7 @@ def examples_star_json(name, match):
                 '../externs/oli.js',
                 '../externs/proj4js.js',
                 '../externs/tilejson.js',
+                '../externs/html5.js',
             ],
         })
         with open(t.name, 'w') as f: