Replace `undefined` equality checks with `defined` function.

There are some errors in the conversion that still need to be addressed.
Fix remaining errors, all specs now pass.
Fix a few more find/replace errors.
Changes after review.
Merge branch 'master' into issue893

Conflicts:
	Source/Core/buildModuleUrl.js
Merge branch 'master' into issue893

Conflicts:
	Apps/CesiumViewer/CesiumViewer.js
	Source/DynamicScene/DataSourceDisplay.js
	Source/Widgets/Viewer/viewerDragDropMixin.js
Fix bad checks due to browser differences.
Fix a few more bad checks.
Switch the last few places to use defined.
Clean up specs, fix one last bad check.
Merge pull request #1026 from AnalyticalGraphicsInc/issue893

Fix #893