In HTML5 Canvas documents, you must assign a linkage name in the Linkage column in the Library.
Any symbol in the Library is a property from the global lib object that is automatically created by Animate when the content is published. To instantiate a symbol at runtime, you must use the linkage name as the constructor method name. Like this:
var platform = new lib.Platform();
platform.x = canvas.width * 0.5;
platform.y = canvas.height * 0.5;