Avoid fatal error when no warnings exist

Author: jcollins-g

lib/src/warnings.dart

@@ -256,12 +256,14 @@ class PackageWarningCounter {
   }
 
   int get errorCount {
+    if (_warningCounts.isEmpty) return 0;
     return _warningCounts.keys
         .map((w) => options.asErrors.contains(w) ? _warningCounts[w] : 0)
         .reduce((a, b) => a + b);
   }
 
   int get warningCount {
+    if (_warningCounts.isEmpty) return 0;
     return _warningCounts.keys
         .map((w) =>
             options.asWarnings.contains(w) && !options.asErrors.contains(w)

testing/test_package_small/pubspec.lock

@@ -0,0 +1,5 @@
+# Generated by pub
+# See http://pub.dartlang.org/doc/glossary.html#lockfile
+packages: {}
+sdks:
+  dart: any