Support parameter metadata.

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 516cc7aa56f04a9f41ae724cb7d37c0f00a78620..03c48e5052df4c06645191226d6649e00b964245 100644
--- a/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
+++ b/sdk/lib/_internal/compiler/implementation/tree/prettyprint.dart
@@ -410,6 +410,14 @@ class PrettyPrinter implements Visitor {
     visitNodeWithChildren(node, "While");
   }
 
+  visitMetadata(Metadata node) {
+    openNode(node, "Metadata", {
+      "token": node.token
+    });
+    visitChildNode(node.expression, "expression");
+    closeNode();
+  }
+
   visitNode(Node node) {
     unimplemented('visitNode', node: node);
   }