This issue was originally filed by dylan.kyle.p...@gmail.com

https://code.google.com/p/dart/source/browse/branches/bleeding_edge/dart/pkg/polymer/lib/src/build/runner.dart

/** Collects different parameters needed to configure and run barback. /
class BarbackOptions {
  /* Other code **/

  /**
   * Whether to print error messages using a json-format that tools, such as the
   * Dart Editor, can process.
   */
  final String machineFormat;

  BarbackOptions(this.phases, this.outDir, {currentPackage, packageDirs,
      this.transformTests: false, this.transformPolymerDependencies: false,
      this.machineFormat: false})
      : currentPackage = (currentPackage != null
          ? currentPackage : readCurrentPackageFromPubspec()),
        packageDirs = (packageDirs != null
          ? packageDirs : _readPackageDirsFromPub(currentPackage));

}

In this code block, it can be obviously seen that machineFormat is of type String, but the default value given in the constructor is a value of false (obviously not a string type). This error keeps popping up when running my build.dart