Improve the offset, length, and end getters in AstNodeImpl

While this does improve the performance of these getters (by about 5%)
the purpose of this CL is just to clean up the implementation of them.
It bothers my that they (especially `end`) are using such an indirect
way of computing their values.

Change-Id: I384890a9adc108c4c67b906864201edee9aa8598
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/426400
Auto-Submit: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Konstantin Shcheglov <scheglov@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

pkg/analyzer/lib/src/dart/ast/ast.dart

@@ -1053,17 +1053,13 @@ sealed class AstNodeImpl implements AstNode {
       _childEntities.syntacticEntities;
 
   @override
-  int get end => offset + length;
+  int get end => endToken.end;
 
   @override
   bool get isSynthetic => false;
 
   @override
-  int get length {
-    var beginToken = this.beginToken;
-    var endToken = this.endToken;
-    return endToken.offset + endToken.length - beginToken.offset;
-  }
+  int get length => end - offset;
 
   /// The properties (tokens and nodes) of this node, with names, in the order
   /// in which these entities should normally appear, not necessarily in the
@@ -1071,10 +1067,7 @@ sealed class AstNodeImpl implements AstNode {
   Iterable<ChildEntity> get namedChildEntities => _childEntities.entities;
 
   @override
-  int get offset {
-    var beginToken = this.beginToken;
-    return beginToken.offset;
-  }
+  int get offset => beginToken.offset;
 
   @override
   AstNode? get parent => _parent;