build_test 0.4.0

Updates to work with build version 0.7.0.

New Features

• The testBuilder method now accepts List<int> values for both
  sourceAssets and outputs.
• The checkOutputs method is now public.

Breaking Changes

• The testBuilder method now requires a RecordingAssetWriter instead of
  just an AssetWriter for the writer parameter.
• If a Matcher is provided as a value in outputs, then it will match against
  the same value that was written. For example if your builder uses
  writeAsString then it will match against that string. If you use
  writeAsBytes then it will match against those bytes. It will not
  automatically convert to/from bytes and strings.
• Deleted the makeAsset and makeAssets methods. There is no more Asset
  class so these don't really have any value any more.
• The signature of addAssets has changed to
  void addAssets(Map<AssetId, dynamic> assets, InMemoryAssetWriter writer).
  Values of the map may be either String or List<int>.
• InMemoryAssetReader#assets and InMemoryAssetWriter#assets have changed to
  a type of Map<AssetId, DatedValue> from a type of
  Map<AssetId, DatedString>. DatedValue has both a stringValue and
  bytesValue getter.
• InMemoryAssetReader and InMemoryAssetWriter have been updated to implement
  the new AssetReader and AssetWriter interfaces (see the build package
  CHANGELOG for more details).
• InMemoryAssetReader#cacheAsset has been changed to two separate methods,
  void cacheStringAsset(AssetId id, String contents) and
  void cacheBytesAsset(AssetId id, List<int> bytes).
• The equalsAsset matcher has been removed, since there is no more Asset
  class.