anchor.diff:  
     TestResult run() {
       TestResult result = runCommand([precompiler, fileName, snapshot], env);
  -    if (result.code != ResultCode.success) {
  +    if (result.exitCode != 0) {
         return result;
       }
       return runCommand([dart, snapshot], env);
  
positive.code:   TestResult run() {
      TestResult result = runCommand(
          [generate, '--gen-bytecode', platform, '-o', dill, fileName], env);
      if (result.code != ResultCode.success) {
        return result;
      }
      return runCommand(cmd, env);
  
negative.code: const sigkill = 9;
  const timeout = 60; // in seconds
  
  // Status of divergence report.
  enum ReportStatus { reported, ignored, rerun, no_divergence }
  
  /// Result of running a test.
  class TestResult {
    const TestResult(this.output, this.stderr, this.exitCode);
    final String output;
    final String stderr;
    final int exitCode;
  }
  
  /// Command runner.
  