Support for imports from isomorphic scripts

[jonathandewitt-dev]

As pointed out in #230, imports on isomorphic scripts fail without any handlers for server-side code. This PR addresses that concern.

EDIT: It's worth noting that the diffs picked up far more than I actually changed. Feel free to confirm, but for sanity, here's the only difference:

var dialogPolyfill = {};

// This condition assures imports don't break for isomorphic scripts,
// while also preventing unexpected uses of this package on the server side.
if (typeof window === 'undefined') {
 function getServerSideThrower(name, isProp) {
   var error = '';
   if (isProp) {
     error = 'Cannot access property `dialogPolyfill.' + name + '` on the server side.';
   } else {
     error = 'Cannot execute `dialogPolyfill.' + name + '()` on the server side.';
   }
   return function() {
     throw new Error(error);
   }
 }
 dialogPolyfill.registerDialog = getServerSideThrower('registerDialog');
 dialogPolyfill.forceRegisterDialog = getServerSideThrower('forceRegisterDialog');
 dialogPolyfill.reposition = getServerSideThrower('reposition');
 dialogPolyfill.isInlinePositionSetByStylesheet = getServerSideThrower('isInlinePositionSetByStylesheet');
 dialogPolyfill.needsCentering = getServerSideThrower('needsCentering');
 dialogPolyfill.DialogManager = getServerSideThrower('DialogManager');
 Object.defineProperties(dialogPolyfill, {
   dm: { get: getServerSideThrower('dm', true) },
   formSubmitter: { get: getServerSideThrower('formSubmitter', true) },
   imagemapUseValue: { get: getServerSideThrower('imagemapUseValue', true) }
 });
} else {
 /* ... the original contents here ... */
}

export default dialogPolyfill;

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

For more information, open the CLA check for this pull request.

[jakec-dev]

Can this please be merged