Fix warnings in dart2js.

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

Index: dart/lib/compiler/implementation/tree/unparser.dart
diff --git a/dart/lib/compiler/implementation/tree/unparser.dart b/dart/lib/compiler/implementation/tree/unparser.dart
index c246dc7e6bfc7eb3094067502fb8950c8b4bc811..1295dea72266f45a769fb46c4d0e597f5c853ce7 100644
--- a/dart/lib/compiler/implementation/tree/unparser.dart
+++ b/dart/lib/compiler/implementation/tree/unparser.dart
@@ -551,4 +551,40 @@ class Unparser implements Visitor {
     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.
+  }
 }