Fix warnings in dart2js.

dart/lib/compiler/implementation/tree/unparser.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 String unparse(Node node) {
   Unparser unparser = new Unparser();
   unparser.unparse(node);
   return unparser.result;
 }
 
@@ skipping 533 matching lines @@
     addToken(node.partKeyword);
     addToken(node.ofKeyword);
     visit(node.name);
     add(node.getEndToken().value);
   }
 
   visitCombinator(Combinator node) {
     addToken(node.keywordToken);
     visit(node.identifiers);
   }
+
+  visitNode(Node node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitExpression(Expression node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitLibraryTag(LibraryTag node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitLiteral(Literal node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitLoop(Loop node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitPostfix(Postfix node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitPrefix(Prefix node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitStatement(Statement node) {
+    throw 'internal error'; // Should not be called.
+  }
+
+  visitStringNode(StringNode node) {
+    throw 'internal error'; // Should not be called.
+  }
 }