Use packageAt:ifAbsent: in chunk exporter.

#packageAt: is deprecated.

src/DOMite.st

@@ -1,5 +1,5 @@
 Smalltalk createPackage: 'DOMite'!
-(Smalltalk packageAt: 'DOMite') imports: {'amber_core/Platform-DOM'}!
+(Smalltalk packageAt: 'DOMite' ifAbsent: [ self error: 'Package not created: DOMite' ]) imports: {'amber_core/Platform-DOM'}!
 ProtoStream subclass: #Domite
 	instanceVariableNames: 'element reference'
 	package: 'DOMite'!