Better error messages; fix for serialization with StandardJsonPlugin

• Add custom Error classes: BuiltValueNullFieldError, BuiltValueMissingGenericsError and BuiltValueNestedFieldError. These provide clearer error messages on failure. In particular, errors in nested builders now report the enclosing class and field name, making them much more useful.
• Fix serialization when using polymorphism with StandardJsonPlugin.

Bump quiver and built_collection versions

Allow quiver 27 and built_collection 3.

Fix for generation when using summaries

This will not affect anyone using the standard (pub) build.

Many features, improvements and fixes

New features:

• Add serialize field to @BuiltValueField. Use this to tag fields to skip
  when serializing.
• Add wireName field to @BuiltValue and @BuiltValueField. Use this to
  override the wire name for classes and fields when serializing.
• Add @BuiltValueEnum and @BuiltValueEnumConst annotations for specifying
  settings for enums. Add wireName field to these to override the wire names
  in enums when serializing.
• Add support for polymorphism to StandardJsonPlugin. It will now specify
  type names as needed via a discriminator field, which by defualt is
  called $. This can be changed in the StandardJsonPlugin constructor.
• Add BuiltListAsyncDeserializer. It provides a way to deserialize large
  responses without blocking, provided the top level serialized type is
  BuiltList.
• Add built in serializer for Uri.

Improvements:

• Allow declaration of multiple Serializers in the same file.
• Explicitly disallow private fields; fail with an error during generation if
  one is found.
• Improve error message for field without type.

Fixes:

• Fix generated builder when fields hold function types.
• Fix checks and generated builder when manually maintained builder has
  generics.
• Fix name of classes generated from a private class.
• Fix builder and serializer generation when importing with a prefix.

Support skipping fields for comparison

• New annotation, BuiltValueField, for configuring fields. First
  setting available is compare. Set to false to ignore a particular field
  for operator== and hashCode.
• Generator now has a const constructor.

Fix for built_collection 2.0.0

The built_collection package now has separate interfaces and implementations, which changes the runtimeType of collection instances. Most people don't need to worry about this, but built_value now needs to be a bit smarter about serialization.

Allow quiver 0.26

Merge pull request #259 from davidmorgan/bump-quiver

Allow quiver 0.26. Release 4.3.3.

Fix generation with multiple levels of inheritance

v4.3.1 introduced a NPE when you have multiple levels of inheritance of interfaces; now fixed.

Fix generation with mixins

Support implementing an interface and at the same time providing a mixin that implements part of the interface.

Now, for such fields, no code is generated; they are concrete fields.

Support serializing Int64

Serialization now supports Int64 from package:fixnum.