First pass at import feature

The core functionality is in place. Still have some minor UI
and workflow stuff outstanding, but this is in good shape!

Paired with @terrasea
Reverse import order so that projects retain export order

Paired with @terrasea.

Fixes #64
Open first imported project on import

Paired with @terrasea

Fixes #65
Switch to dart:convert for JSON parsing
Upgrade to latest js-interop
Fix build for deployment
Ignore seemingly useless error

#68
Need to download content_shell in order to run tests.
Initial spike for drop code files

Working. Still need iterate over files.

Paired with @tmk.

#59
Prevents clobber of existing projects when importing

Fixes #62
Merge pull request #69 from terrasea/master

Prevents clobber of existing projects when importing
Reuse import project code to prevent clobbering

Ensure that existing projects are not overwritten when dropping or when importing.

Paired with @mr170

Fixes #71
Add Michael Reynolds to the list of contributors
Alert dialog if user imports non JSON

fixes #70
Merge pull request #72 from terrasea/master

Alert dialog if user imports non JSON
Add James Hurford as core collaborator
Distinguish between export file and regular file uploads

When performed via drag and drop.

Paired with @terrasea

Fixes #67
Minor re-org of the main menu

Move Remove item up with other single project actions. Add divider around
multi-project import/export items.

Paired with @alexgchacon

Fixes #66
Remove unnecessary TODO

We're not going to handle multiple uploads -- at least until we have
a way to represent mutliple items in the UI immediately after the
upload.

Paired with @alexgchacon

Fixed #59
Spike a something's new indicator for main menu button

Paired with @alexgchancon

#73