问TravisCI - ERR :包flute_music_player需要Flutter版本^0.1.2，但是当前的SDK是0.0.0-未知

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    $ ./flutter/bin/flutter test
    Running "flutter packages get" in tumultus...             
    Package flute_music_player requires Flutter SDK version ^0.1.2 but the current SDK is 0.0.0-unknown.
    ---- Log transcript ----
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---- End log transcript ----
pub get failed (1)
The command "./flutter/bin/flutter test" exited with 1.
store build cache
nothing changed, not updating cache
Done. Your build exited with 1.

os:
  - linux
sudo: false
addons:
  apt:
    # Flutter depends on /usr/lib/x86_64-linux-gnu/libstdc++.so.6 version GLIBCXX_3.4.18
    sources:
      - ubuntu-toolchain-r-test # if we don't specify this, the libstdc++6 we get is the wrong version
    packages:
      - libstdc++6
      - fonts-droid
before_script:
  - git clone https://github.com/flutter/flutter.git -b alpha --depth 1
  - ./flutter/bin/flutter doctor
script:
  - ./flutter/bin/flutter test
cache:
  directories:
    - $HOME/.pub-cache