Get a PolygonMorph out of the Supplies flap. Bring up its halo, and
choose "make open polygon" from its menu.
Now bring up a viewer and change its gradientFill to true.
You get this walkback:

23 June 2002 4:57:10 pm

VM: unix - Squeak3.3alpha of 30 January 2002 [latest update: #4881]
Image: Squeak3.2gamma [latest update: #4889]

BorderStyle class(Object)>>error:
	Receiver: BorderStyle
	Arguments and temporary variables: 
		aString: 	'MessageNotUnderstood: a GradientFillStyle'
	Receiver's instance variables: 
		superclass: 	Object
		methodDict: 	a MethodDictionary(#=->a CompiledMethod (159)
#baseColor->a Compile...etc...
		format: 	2
		instanceVariables: 	nil
		organization: 	('initialize' releaseCachedState)
('accessing' baseColor baseColo...etc...
		subclasses: 	#(SimpleBorder)
		name: 	#BorderStyle
		classPool: 	a Dictionary(#Default->a BorderStyle )
		sharedPools: 	nil
		environment: 	a SystemDictionary(lots of globals)
		category: 	nil

[] in WorldState>>displayWorldSafely:
	Arguments and temporary variables: 
		aWorld: 	a PasteUpMorph(768) [world]
		oldHandler: 	nil
		errCtx: 	PolygonMorph(BorderedMorph)>>borderStyle
		errMorph: 	a PolygonMorph<Polygon>(2531)
		err: 	'MessageNotUnderstood: a GradientFillStyle'
		rcvr: 	BorderStyle

[] in BlockContext>>ifError:
	Arguments and temporary variables: 
		errorHandlerBlock: 	[] in WorldState>>displayWorldSafely:
		lastHandler: 	nil
		val: 	nil
		activeProcess: 	a Process in Debugger
class>>openContext:label:contents:
		errBlockArgs: 	2
		aString: 	nil
		aReceiver: 	nil
		ex: 	a MessageNotUnderstood

MessageNotUnderstood(Exception)>>handlerAction
	Receiver: a MessageNotUnderstood
	Arguments and temporary variables: 
		na: 	1
		handler: 	[] in BlockContext>>ifError:
		returnValue: 	nil
	Receiver's instance variables: 
		messageText: 	nil
		initialContext: 	BorderStyle class(Object)>>doesNotUnderstand:
		resignalException: 	nil
		handlerContext: 	BlockContext>>on:do:
		tag: 	nil
		activeHandler: 	nil
		message: 	a Message with selector: a GradientFillStyle and arguments:
#()

--- The rest of the stack ---
MessageNotUnderstood(Exception)>>signal
BorderStyle class(Object)>>doesNotUnderstand:
PolygonMorph(BorderedMorph)>>borderStyle
PolygonMorph>>drawBorderOn:usingEnds:
[] in PolygonMorph>>drawClippedBorderOn:usingEnds:
FormCanvas>>clipBy:during:
PolygonMorph>>drawClippedBorderOn:usingEnds:
PolygonMorph>>drawOn:
FormCanvas(Canvas)>>draw:
FormCanvas(Canvas)>>drawMorph:
[] in PolygonMorph(Morph)>>fullDrawOn:
FormCanvas>>roundCornersOf:in:during:
FormCanvas(Canvas)>>roundCornersOf:during:
PolygonMorph(Morph)>>fullDrawOn:
FormCanvas(Canvas)>>fullDraw:
FormCanvas(Canvas)>>fullDrawMorph:
[] in WorldState>>drawWorld:submorphs:invalidAreasOn:
Rectangle>>allAreasOutsideList:startingAt:do:
Rectangle>>allAreasOutsideList:do:
[] in WorldState>>drawWorld:submorphs:invalidAreasOn:
Array(SequenceableCollection)>>do:
WorldState>>drawWorld:submorphs:invalidAreasOn:
[] in WorldState>>displayWorld:submorphs:
FormCanvas>>roundCornersOf:in:during:
FormCanvas(Canvas)>>roundCornersOf:during:
WorldState>>displayWorld:submorphs:
PasteUpMorph>>privateOuterDisplayWorld
PasteUpMorph>>displayWorld
[] in WorldState>>displayWorldSafely:
BlockContext>>on:do:
BlockContext>>ifError:
WorldState>>displayWorldSafely:
WorldState>>doOneCycleNowFor:
WorldState>>doOneCycleFor:
PasteUpMorph>>doOneCycle
[] in Project class>>spawnNewProcess
[] in BlockContext>>newProcess