Add SyntacticEntity, an interface shared by AstNode and Token.

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

Index: pkg/analyzer/lib/dart/ast/ast.dart
diff --git a/pkg/analyzer/lib/dart/ast/ast.dart b/pkg/analyzer/lib/dart/ast/ast.dart
index da182240d6f00a10da4f22f622ee380cd5dc2ec0..1e6b12ebee9d8a2d765a668057648073ffabd980 100644
--- a/pkg/analyzer/lib/dart/ast/ast.dart
+++ b/pkg/analyzer/lib/dart/ast/ast.dart
@@ -510,7 +510,7 @@ abstract class AssignmentExpression extends Expression {
  *
  * Clients may not extend, implement or mix-in this class.
  */
-abstract class AstNode {
+abstract class AstNode implements SyntacticEntity {
   /**
    * An empty list of AST nodes.
    */
@@ -536,7 +536,7 @@ abstract class AstNode {
    * (either AST nodes or tokens) that make up the contents of this node,
    * including doc comments but excluding other comments.
    */
-  Iterable /* AstNode | Token */ get childEntities;
+  Iterable<SyntacticEntity> get childEntities;
 
   /**
    * Return the offset of the character immediately following the last character
@@ -546,6 +546,7 @@ abstract class AstNode {
    * equivalent to the node's offset (because the length is zero (0) by
    * definition).
    */
+  @override
   int get end;
 
   /**
@@ -560,15 +561,10 @@ abstract class AstNode {
    */
   bool get isSynthetic;
 
-  /**
-   * Return the number of characters in the node's source range.
-   */
+  @override
   int get length;
 
-  /**
-   * Return the offset from the beginning of the file to the first character in
-   * the node's source range.
-   */
+  @override
   int get offset;
 
   /**