Support symbol literals.

sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart

Index: sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
diff --git a/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart b/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
index 60f48078f507e1d66fcaeef04fec2e89883d3872..1126d11564426841d9cfc7c9a9469f90b06484a1 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
@@ -367,6 +367,13 @@ class PrettyPrinter implements Visitor {
     visitNodeWithChildren(node, "SwitchStatement");
   }
 
+  visitSymbolLiteral(SymbolLiteral node) {
+    openNode(node, "SymbolLiteral");
+    visitChildNode(node.operator, "operator");
+    visitChildNode(node.identifiers, "identifiers");
+    closeNode();
+  }
+
   visitThrow(Throw node) {
     visitNodeWithChildren(node, "Throw");
   }