anchor.diff:      _dependencies = toAttrSet (map verifyNodePackage deps);
       _optionalDependencies = toAttrSet (map verifyNodePackage optionalDependencies);
       _peerDependencies = toAttrSet (map verifyNodePackage peerDependencies);
  -    _devDependencies = if !doCheck then {}
  +    # Dev dependencies will only be included if requested.
  +    _devDependencies = if !includeDevDependencies then {}
                          else toAttrSet (map verifyNodePackage devDependencies);
   
       # Depencencies we need to propagate (all except devDependencies)
  
positive.code: 
  in
  
  if doCheck && (devDependencies == null)
  then throw ("${uniqueName}: Can't run tests because devDependencies have " +
              "not been defined.")
  else
  
  let
  
negative.code:   # installed contingent on successfully running tests.
    doCheck ? false,
  
    # If true, devDependencies will be added to the packages to the
    # build environment. By default, this is true whenever doCheck is true.
    includeDevDependencies ? doCheck,
  
    # Bash command to run package tests.
    checkPhase ? defaultCheckPhase,
  
    # Additional flags passed to npm install. A list of strings.
    extraNpmFlags ? [],
  
    # Build inputs to propagate in addition to nodejs and non-dev dependencies.
    propagatedBuildInputs ? [],
  