Fix to generated code

Fix generated serialization code when a manually declared builder causes a field to not use a nested builder.

Workaround for analyzer issue

Workaround for analyzer issue when implementing multiple classes that
use @BuiltValue(instantiable: false).

Relax restriction on `extends`

• Relax restriction on extends to allow for one special case: sharing of
  code between built_value and const classes. The base class in question
  must be abstract, have no fields, have no abstract getters and must not
  implement operator==, hashCode or toString.

Allow quiver 0.28

Merge pull request #334 from davidmorgan/bump-quiver

Allow quiver 0.28; release 5.0.1.

Restrict using built_value in unsupported ways

This release makes built_value actively block use in three unsupported ways. If you do need any of these to work, please file an issue and we'll look into it.

• Use of extends is prohibited; use implements or extends Object with instead.
• Importing package:built_value/built_value.dart with show or as is prohibited; this hides symbols from the generated code.
• Use of SDK collections in fields is prohibited; they are mutable and break the built_value guarantees.

Any problems please file an issue here: https://github.com/google/built_value.dart/issues

Allow hand-coded base builders

For classes using @BuiltValue(instantiate: false, you can now hand-code the builder. This allows you to customize the base builder.

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.