I spent quite a while debugging this, until I finally asked Scott Blum, a GWT Engineer.  Scott merely asked a question:  "Was the array created in the same window in which you're testing it?"

What Scott knew that I did not, is that the JavaScript classes (like Array) corresponding to the primitive types are constructed along with the window object.  Because JavaScript is a prototype-oriented language, the "classes" are really just object instances with special names.  These two issues combine to reveal a subtle but important issue: the Array objects from two different windows are not the same object!  The expression "x instanceof y" in JavaScript boils down to something like this pseudocode:  "if the 'prototype' property of 'x' is the same object as 'y', return true, else return false".

At this point, you may be wondering how multiple windows entered the discussion.  The key is the $wnd variable, which points to the hidden iframe in which the application's GWT code is loaded.  The DOM object in GWT, however, points to the parent window's document object; after all, your application code is interested in manipulating the browser window, not GWT's hidden iframe.  As a result, in the code above, the <script> tag is added to the parent window, while the code using it resides in a different iframe.  This means that the object is created in a window different from where the "instanceof" checks are made, thus causing the issue above.