Change assert implementation to not depend on a top-level function called 'assert'.

pkg/dartdoc/dartdoc.dart

Index: pkg/dartdoc/dartdoc.dart
diff --git a/pkg/dartdoc/dartdoc.dart b/pkg/dartdoc/dartdoc.dart
index f295db4e287cd966fa2d0b264386ae28b145f48c..3aec3503a4e5fbffdbb93547b9646e5f00bfb62d 100644
--- a/pkg/dartdoc/dartdoc.dart
+++ b/pkg/dartdoc/dartdoc.dart
@@ -1727,3 +1727,12 @@ class Dartdoc {
   }
 }
 
+/**
+ * Used to report an unexpected error in the DartDoc tool or the
+ * underlying data
+ */
+class InternalError {
+  final String message;
+  const InternalError(this.message);
+  String toString() => "InternalError: '$message'";
+}