Test fix

• Test fix for new analyzer hint.

Remove obsolete restrictions

5.1.0

• Allow collections with nullable types, for example BuiltList<T?>.
• Allow key/value types to be dynamic. This can be useful occasionally, and
  with Dart 2 and null safety it's much harder to use dynamic by accident.

Stable null safe release

v5.0.0

Release 5.0.0.

Null safe `built_collection`

• Migrate to NNBD.
• Deps on package:collection and package:quiver have been removed.
• Multimap builders no longer have addAll methods that accept multimaps from quiver. But, the constructors still work with quiver multimaps.

Add an example

• Add an example folder with some example code.

Internal cleanup

• Internal: cleanup for pedantic v1.9.0 lints.

Extension methods

• Add extensions methods: List.build, Set.build, Map.build,
  Iterable.toBuiltList and Iterable.toBuiltSet. Requires SDK 2.6.0.
• Cleanup: remove unnecessary new and const.

Bug fix to `ListMultimapBuilder.[]`

• Bug fix: ListMultimapBuilder.[] no longer ignores modifications under some
  circumstances.

Bug fix to `ListBuilder` `first` and `last`

• Bug fix: ListBuilder first and last setters can no longer modify the
  underlying collection.

Performance improvements

• Add benchmark for VM and dart2js.
• Stop doing explicit type checks on elements; in Dart 2 these are implied.
• Modify methods on ListBuilder and SetBuilder that take Iterable
  elements so they only iterate over the iterable once. Improves performance
  when the iterable are slow/lazy.