Enable eslint "no-undef" rule

[Robbert]

Currently we have defined { "no-undef": ["off"] } in .eslintrc because there are dependencies between files but the files are checked as standalone scripts. This results in hundreds of "no-undef" errors, because constructors are extended with properties and methods in different files than were the constructor is defined. This is especially the case in the files were browser specific extensions to DOM APIs are defined. For example:

webkit_dom.js

Element.prototype.scrollIntoViewIfNeeded = function(opt_center) {};

Above will result in an error because Element is undefined. That's true, because it is defined in another file: w3c_dom1.js:

/**
 * @constructor
 * @extends {Node}
 * @see http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/level-one-core.html#ID-745549614
 */
function Element() {}

It would still be useful to catch unintended undefined references, which can be caused easily by a typo, so we need to find a way to resolve the dependencies before running eslint.

• Clarify the subject of this issue, for future reference
• Define and resolve dependencies between extern files.
• Enable no-undef rule in .eslintrc