Fix build script for google closure and CommonJS
Fix build script for google closure and CommonJS

When assigning a value to `process.env` it'll be implicitly converted to
string and therefore inline environment variables will always be
transformed even if `env.NODE_ENV` should be `undefined`.
The documentation suggests to use `delete`.
@see https://github.com/nodejs/node/commit/396e4b9199
google/incremental-dom,google/incremental-dom,jridgewell/incremental-dom,PolymerLabs/incremental-dom,sparhami/incremental-dom,jridgewell/incremental-dom,jridgewell/incremental-dom,PolymerLabs/incremental-dom,sparhami/incremental-dom,sparhami/incremental-dom,sparhami/incremental-dom,google/incremental-dom,PolymerLabs/incremental-dom