Advanced features, formatting and fixes

• Add @BuiltValue(generateBuilderOnSetField: true) which provides a way to
  listen for set calls on generated builders.
• Add @BuiltValueEnumConst(fallback: true) as a way to mark an enum const
  as the fallback when valueOf or deserialization fails.
• Add @BuiltValueSerializer(serializeNulls: true as a way to modify the wire
  format to explicitly contain null values.
• Make it possible to merge Serializers instances: add a builderFactories
  getter that returns installed builder factories.
• Use new Function syntax everywhere.
• Bug fix: only generate builder factories for fields that are Built
  types or built_collection collections.

Allow `analyzer` 0.35.0

Merge pull request #602 from davidmorgan/release-6-3-2

Release 6.3.2.

Fix serialization corner case

• Fix BuiltList serialization when using StandardJsonPlugin with unspecified type and when list length is 1.

Builder flexibility and minor features

• Allow custom builders to use setter/getter pairs instead of normal fields.
• Add an option to @BuiltValue to turn off auto instantiation of nested
  builders.
• Add @BuiltValueSerializer annotation which gives the option to specify a
  custom serializer for a class.
• Make it possible to merge Serializers instances: add a serializers
  getter that returns the installed serializers.
• Add serializer for RegExp fields.
• Allow analyzer 0.34.0.

Comparable builders; serializable `Duration`; fixes

New features:

• Add an option to @BuiltValue to generate comparable builders.
• Add serializer for Duration fields.
• Add serializerForType and serializerForWireName methods to Serializers.

Improvements:

• Add ignore for avoid_as lint to generated code.
• Put ignored lints on a single line at the end of the generated output.
• Stop checking for import of built_value.dart when EnumClass is used; this
  was expensive.

Fixes:

• Fix tests following changes to source_gen error output.
• Fix generation when new mixin declarations are used.
• Support dollar signs in enum value names.
• Fix nested collections when using a custom builder.

Switch to new analyzer API in version `0.33.3`.

Merge pull request #530 from davidmorgan/analyzer-patch

Update for analyzer 0.33.3.

Bump versions of `analyzer`, `analyzer_plugin`, `build`, `build_runner`

Merge pull request #526 from davidmorgan/release-6-1-5

Release 6.1.5.

Minor fixes, `build` version

• Allow polymorphic base classes to omit implementing Built while still
  implementing any other interface(s).
• Allow the dollar character in wireName settings.
• Allow build version 1.0.

Improvements to built_value_test

• Add built_value_test support for remaining built collections.

Minor fixes

• Fix generated operator== when a type uses generic functions.
• Fix generated code for curly_braces_in_control_flow lint.