…evaluated if used

validated via type check system.  Additionally adds document click
listener which will expand href of anchor target.

Added replacement service to experiments.js which ensures it is fully
typed check.  Fixed numerous errors in other files to ensure type check
executed on modified portions.  Managed to get type check failures down
to the following within the file which are mostly unrelated.  At some
point this quarter, AMP will be moving to full type checking.  The
following changes were not included as they were only to facilitate type
checks.

Changes not staged for commit:
       modified:   src/animation.js
       modified:   src/curve.js
       modified:   src/experiments.js
       modified:   src/time.js
       modified:   src/transition.js
       modified:   src/url-replacements.js
       modified:   src/viewer.js
       modified:   src/viewport.js
       modified:   src/vsync.js

src/service/url-replacements-impl.js:68: ERROR - assignment to property getAccessService_ of UrlReplacements$$module$src$service$url_replacements_impl
found   : function (Window): Promise<(AccessService|null)>
required: function ((Window|null)): (Promise<(AccessService|null)>|null)
    this.getAccessService_ = accessServiceForOrNull;
    ^

src/service/url-replacements-impl.js:116: ERROR - inconsistent return type
found   : Promise<string>
required: Promise<(string|undefined)>
      return viewerFor(this.win_).getReferrerUrl();
             ^

src/service/url-replacements-impl.js:188: ERROR - Property get never defined on UserNotificationManager
          return service.get(opt_userNotificationId);
                         ^

src/service/url-replacements-impl.js:192: ERROR - Property get never defined on Cid
        return cid.get({
                   ^

src/service/url-replacements-impl.js:204: ERROR - restricted index type
found   : *
required: string
        user.assert(variants[experiment] !== undefined,
                             ^

src/service/url-replacements-impl.js:207: ERROR - restricted index type
found   : *
required: string
        const variant = variants[experiment];
                                 ^

src/service/url-replacements-impl.js:331: ERROR - inconsistent return type
found   : (Promise<(AccessService|null)>|null)
required: Promise<(string|undefined)>
      return this.getAccessValue_(accessService => {
             ^

src/service/url-replacements-impl.js:332: ERROR - Property getAccessReaderId never defined on AccessService
        return accessService.getAccessReaderId();
                             ^

src/service/url-replacements-impl.js:340: ERROR - inconsistent return type
found   : (Promise<(AccessService|null)>|null)
required: Promise<(string|undefined)>
      return this.getAccessValue_(accessService => {
             ^

src/service/url-replacements-impl.js:341: ERROR - Property getAuthdataField never defined on AccessService
        return accessService.getAuthdataField(field);
                             ^

src/service/url-replacements-impl.js:347: ERROR - inconsistent return type
found   : Promise<string>
required: Promise<(string|undefined)>
      return viewerFor(this.win_).getViewerOrigin();
             ^

src/service/url-replacements-impl.js:353: ERROR - Property getTotalEngagedTime never defined on Activity
        return activity.getTotalEngagedTime();
                        ^

src/service/url-replacements-impl.js:415: ERROR - actual parameter 1 of module$src$event_helper.loadPromise does not match formal parameter
found   : Window
required: Element
      return loadPromise(this.win_).then(() => {
                         ^

src/service/url-replacements-impl.js:491: ERROR - assignment
found   : {async: (function (...*): Promise<(string|undefined)>|undefined), sync: (function (...*): (string|undefined)|null)}
required: {async: function (...*): Promise<(string|undefined)>, sync: function (...*): (string|undefined)}
    this.replacements_[varName] =
    ^

src/service/url-replacements-impl.js:684: ERROR - restricted index type
found   : string
required: number
        if (allKeys[key] === undefined) {
                    ^

src/service/url-replacements-impl.js:807: ERROR - inconsistent return type
found   : *
required: UrlReplacements$$module$src$service$url_replacements_impl
  return getService(window, 'url-replace', () => {