build 0.7.0

A number of changes to the apis, primarily to support reading/writing as bytes,
as this is going to inevitably be a required feature. This will hopefully be the
last breaking change before the 1.0 release, but it is a fairly large one.

New Features

• The AssetWriter class now has a
  Future writeAsBytes(AssetId id, List<int> bytes) method.
• The AssetReader class now has a Future<List<int>> readAsBytes(AssetId id)
  method.
• You no longer need to call Resolver#release on any resolvers you get from
  a BuildStep (in fact, the Resolver interface no longer has this method).
• There is now a BuildStep#resolver getter, which resolves the primary input,
  and returns a Future<Resolver>. This replaces the BuildStep#resolve
  method.
• Resolver has a new isLibrary method to check whether an asset is a Dart
  library source file before trying to resolve it's LibraryElement

Breaking Changes

• The Asset class has been removed entirely.
• The AssetWriter#writeAsString signature has changed to
  Future writeAsString(AssetId id, String contents, {Encoding encoding}).
• The type of the AssetWriterSpy#assetsWritten getter has changed from an
  Iterable<Asset> to an Iterable<AssetId>.
• BuildStep#input has been changed to BuildStep#inputId, and its type has
  changed from Asset to AssetId. This means you must now use
  BuildStep#readAsString or BuildStep#readAsBytes to read the primary input,
  instead of it already being read in for you.
• Resolver no longer has a release method (they are released for you).
• BuildStep#resolve no longer exists, and has been replaced with the
  BuildStep#resolver getter.
• Resolver.getLibrary will now throw a NonLibraryAssetException instead of
  return null if it is asked to resolve an impossible library.

Note: The changes to AssetReader and AssetWriter also affect BuildStep
and other classes that implement those interfaces.