Dart-code: Support launching DevTools with the dark theme

If we add support, it may have to be a user setting in VS Code to let you choose which theme you want?

Dark theme is worth my effort to change user setting manually :)

Dark theme will be default from vNext. There's a new setting that lets you set the theme:

"dart.theme": "light" // or "dark"

This controls both the icons in tooltips and the DevTools theme. If it turns out that people would like to use different themes for VS Code/icons than DevTools, we can add a separate setting in future.

Not totally happy with how the Dark theme looks right now, so undoing this and re-opening, and will re-evaluate before this version ships.

There are two settings in vNext:

• dart.theme (defaults to dark)
• dart.devToolsTheme (defaults to light)

When the Dark Theme is more polished, I'll change the default of dart.devToolsTheme, but for now you can opt-in to it manually.