dart2js: fix crash when annotations have syntax errors (fix #23983)

pkg/compiler/lib/src/scanner/listener.dart

Index: pkg/compiler/lib/src/scanner/listener.dart
diff --git a/pkg/compiler/lib/src/scanner/listener.dart b/pkg/compiler/lib/src/scanner/listener.dart
index 9df843cf7002ad57656119d7367c3f70a1cd9e5b..c3299a3c5eed23f18200e199e261c94cf84b49ea 100644
--- a/pkg/compiler/lib/src/scanner/listener.dart
+++ b/pkg/compiler/lib/src/scanner/listener.dart
@@ -2566,12 +2566,17 @@ class PartialMetadataAnnotation extends MetadataAnnotationX
 
   Node parseNode(DiagnosticListener listener) {
     if (cachedNode != null) return cachedNode;
-    Metadata metadata = parse(listener,
-                              annotatedElement,
-                              declarationSite,
-                              (p) => p.parseMetadata(beginToken));
-    cachedNode = metadata.expression;
-    return cachedNode;
+    var metadata = parse(listener,
+                         annotatedElement,
+                         declarationSite,
+                         (p) => p.parseMetadata(beginToken));
+    if (metadata is Metadata) {
+      cachedNode = metadata.expression;
+      return cachedNode;
+    } else {
+      assert (metadata is ErrorNode);
+      return metadata;
+    }
   }
 
   bool get hasNode => cachedNode != null;