code: function getPeerGlobals(externals = [], bundleType) {
    const peerGlobals = {}
    externals.forEach(name => {
      if (!knownGlobals[name] && bundleType === UNIVERSAL) {
        throw new Error("Cannot build UMD without a global name for: " + name)
      }
      peerGlobals[name] = knownGlobals[name]
    })
    return peerGlobals
  }
docs: Given ['react'] in bundle externals, returns { 'react': 'React' }.