[cfe] Change expression_suite to use embedded sources

This changes that expression_suite tests to embed the needed source
code in the yaml file. Single source and multi sources are supported.
When using a single source, the file name is implicitly "main.dart"
and the "position" and "entry_point" yaml properties are updated to
supports this, meaning that the yaml files should only use file names
when using multiple sources.

Change-Id: Ia6756eb86a7b72dea2187f72b9f2c75c28783a0b
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/237680
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>