Description

Based on a survey of usage, it appears that the ratio is about
3:1 of callers that create StreamBuilder without initial
data to callers that create StreamBuilder with initial data.
Given this, the updated API will break fewer callers if we swap
the semantics of the two constructors:

Before

StreamBuilder(
  @required T initialData,
  ...
)

StreamBuilder.withoutInitialData(
  ...
)

After

StreamBuilder(
  ...
)

StreamBuilder.withInitialData(
  @required T initialData,
  ...
)

This also allows us to provide a softer transition period by
deprecating the initialData argument to the default constructor.
During this deprecation period, null values will be interpreted as
"no initial data".

Related Issues

#34545

Tests

I updated the existing tests to reflect the switch.

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

• Yes, this is a breaking change (though only for users on master). I will update the existing breaking change announcement at https://groups.google.com/forum/#!topic/flutter-announce/H6Od0QdsdrI
• No, this is not a breaking change.