[CFE] Add comments to clarify source encoding and offsets

I mistakenly thought the offset would be the location in the List<int> source.
Hopefully these comments will prevent confusion in the future.

See:
* https://dart-review.googlesource.com/c/sdk/+/124602
* #39271
Change-Id: Iba47b477d085c2a133ccd9aca645fe10906368bc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124591
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Nicholas Shahan <nshahan@google.com>

Author: nshahan

pkg/kernel/lib/ast.dart

@@ -4518,7 +4518,15 @@ class EmptyStatement extends Statement {
 class AssertStatement extends Statement {
   Expression condition;
   Expression message; // May be null.
+
+  /// Character offset in the source where the assertion condition begins.
+  ///
+  /// Note: This is not the offset into the UTF8 encoded `List<int>` source.
   int conditionStartOffset;
+
+  /// Character offset in the source where the assertion condition ends.
+  ///
+  /// Note: This is not the offset into the UTF8 encoded `List<int>` source.
   int conditionEndOffset;
 
   AssertStatement(this.condition,
@@ -7051,6 +7059,7 @@ class _ChildReplacer extends Transformer {
 class Source {
   final List<int> lineStarts;
 
+  /// A UTF8 encoding of the original source file.
   final List<int> source;
 
   final Uri importUri;