[type-scripts-build] fails when styled-components is not an external dependency

[ncphillips]

if styled-components is a peerDependency in the package.json everything works
but if it's a regular dependency than an error is emitted at build time:

$ type-scripts-build
Creact Typescript Package – Build
        Typescript Version:  2.6.2
        Package Name:  @forestryio/forestry-ui
        External Dependencies:  [ '@forestryio/icons', 'react' ]
(node:74562) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): Error: 'default' is not exported by node_modules/is-plain-object/index.js
(node:74562) [DEP0018] DeprecationWarning: Unhandled promise rejections are deprecated. In the future, promise rejections that are not handled will terminate the Node.js process with a non-zero exit code.
✨  Done in 2.59s.

It almost looks like a Typings problem but I'm not sure