Always set the previous token in AstCloner.

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

Index: pkg/analyzer/lib/src/dart/ast/utilities.dart
diff --git a/pkg/analyzer/lib/src/dart/ast/utilities.dart b/pkg/analyzer/lib/src/dart/ast/utilities.dart
index 05ff07f1442b04ef2e3ca3b6c19b8e3e0ee53f42..107a28b1f691721f37965cda94e51ca69c77c802 100644
--- a/pkg/analyzer/lib/src/dart/ast/utilities.dart
+++ b/pkg/analyzer/lib/src/dart/ast/utilities.dart
@@ -42,7 +42,7 @@ class AstCloner implements AstVisitor<AstNode> {
   /**
    * The last cloned token.
    */
-  Token _lastCloned;
+  Token _lastCloned = new Token(TokenType.EOF, -1);

[Brian Wilkerson, 2016/02/18 21:00:34]

The EOF token should be it's own 'previous' token.

 
   /**
    * The offset of the last cloned token.
@@ -952,7 +952,7 @@ class AstCloner implements AstVisitor<AstNode> {
         }
       }
       _clonedTokens[token] = clone;
-      _lastCloned?.setNext(clone);
+      _lastCloned.setNext(clone);
       _lastCloned = clone;
       if (token.type == TokenType.EOF) {
         break;