Fine. Record isSynthetic in manifest items.

Capture whether an element is synthetic in the manifest and make it part
of equality/matching so changes between implicit and explicit
declarations are treated as API-relevant.

Previously the manifest did not distinguish synthetic from declared
members (e.g., default constructors, field-generated accessors, and
variable-generated top-level accessors). Flipping between synthetic and
explicit could reuse stale IDs and interfaces, leading to incorrect link
reuse and dependency tracking.

It might be surprising, but we actually do use `isSynthetic` during
resolution, e.g. to decide whether look at metadata of a getter, or the
field.

This change:
- Adds an `isSynthetic` flag to `ManifestItem` and all subclasses.
- Populates it from `element.isSynthetic` in `fromElement` factories.
- Persists it in the binary format (written after `id`, before metadata)
  and reads it in all `read(...)` constructors.
- Includes it in `ManifestItem.match(...)` so ID stability reflects
  synthetic ↔ explicit transitions.
- Bumps `AnalysisDriver.DATA_VERSION` to 535 to invalidate old caches.

Impact: switching between synthetic and explicit declarations now causes
manifest ID changes and corrects fine-grained invalidation without
affecting unrelated behavior.

Change-Id: I6e4bc83f3f71573c3e6fa73c83857998585cc29d
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/448080
Reviewed-by: Johnni Winther <[email protected]>
Reviewed-by: Paul Berry <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: scheglov

pkg/analyzer/lib/src/dart/analysis/driver.dart

@@ -106,7 +106,7 @@ testFineAfterLibraryAnalyzerHook;
 // TODO(scheglov): Clean up the list of implicitly analyzed files.
 class AnalysisDriver {
   /// The version of data format, should be incremented on every format change.
-  static const int DATA_VERSION = 536;
+  static const int DATA_VERSION = 538;
 
   /// The number of exception contexts allowed to write. Once this field is
   /// zero, we stop writing any new exception contexts in this process.