Icons on Extension blocks should be more exportable

[zoebentley]

Expected Behavior

All icons on extension blocks should be visible when exporting images for tutorials.

Actual Behavior

When exporting SVG of blocks, some extension icons export in a way such that they do not appear in Adobe Illustrator.

Extension icons that don't appear in Adobe Illustrator:

• Makey Makey
• Music
• Pen
• Text-to-Speech
• Video Sensing
• EV3

Extension icons that do appear in Adobe Illustrator:

• Translate
• Micro:bit
• BOOST
• WeDo
• Vernier

Steps to Reproduce

Build a stack containing extension blocks
Export using Paula's method
Open noname.svg in Adobe Illustrator
Note: Some icons are missing

Operating System and Browser

Mac 10.13.6 Chrome