[ddc] Library level expression evaluation for the Dart SDK.

Today, support for expression evaluation in DDC uses a lot of
information.  To properly support compiling expressions in arbitrary
frames, we consume source-maps, module metadata, and full kernel
components at various stages of the process.  This data is already
plumbed and available for non-sdk modules in our debugging systems
(frontend-server, webdev, g3), however it is not available the Dart
SDK module itself.  Because of that, the expression compiler today
rejects all expression evaluation requests when the target is a Dart
SDK library.

Until the fix the availability of this data everywhere, we believe
we can support a limited form of expression evaluation: only
evaluation of libary level expressions. Basically, expressions
where scope data is not necessary and kernel outlines are sufficient to
compile the expression.

Here we introduce logic to recognize the first offset location of a
library, which is a clear indicator that no scope data is needed and
a library level expression evaluation is being requested.

This change is an attempt to help resolve
https://github.com/flutter/devtools/issues/7766.  Note: just like
changes were needed here in the worker and test-compiler files, it
is possible that some change may be necessary in dwds and other
parts of the pipeline.  Additional validation will be needed after
this lands.

Change-Id: Iff081a24ecb05092407279a0a7ed3d38c13cf41d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367981
Reviewed-by: Nicholas Shahan <nshahan@google.com>
Commit-Queue: Sigmund Cherem <sigmund@google.com>