Handle diagnostic messages from analyzer that lack a URI.

pkg/testing/lib/src/analyze.dart

Index: pkg/testing/lib/src/analyze.dart
diff --git a/pkg/testing/lib/src/analyze.dart b/pkg/testing/lib/src/analyze.dart
index 85e4dcc6a46528ccaf974839bb86d9275f2664f9..38324578eb580224d94f9e0b942070cef0c29921 100644
--- a/pkg/testing/lib/src/analyze.dart
+++ b/pkg/testing/lib/src/analyze.dart
@@ -166,7 +166,7 @@ Future<Null> analyzeUris(Uri analysisOptions, Uri packages, List<Uri> uris,
   bool hasOutput = false;
   Set<String> seen = new Set<String>();
   for (AnalyzerDiagnostic diagnostic in diagnostics) {
-    String path = diagnostic.uri.path;
+    String path = diagnostic.uri?.path ?? "<unknown>";
     if (exclude.any((RegExp r) => path.contains(r))) continue;
     String message = "$diagnostic";
     if (seen.add(message)) {