title: build.ninja is not found when building Dart SDK
body: <p>I followed <a href="https://github.com/dart-lang/sdk/wiki/Building#getting-the-source" rel="nofollow noreferrer">the wiki</a> accurately, but when I try to build, it fails:</p>
  
  <pre><code>$ gclient sync
  ... # A lot of syncing here
  Running hooks: 100% (14/14), done.
  $ cd sdk/
  $ ./tools/build.py --mode release --arch x64 create_sdk
  ninja -C out/ReleaseX64 create_sdk
  ninja: error: loading 'build.ninja': No such file or directory
  BUILD FAILED
  $ ls out/ReleaseX64
  args.gn build.ninja build.ninja.d dart-sdk obj toolchain.ninja
  </code></pre>