Add configuration support for choosing test paths.

lib/src/runner/configuration/values.dart

Index: lib/src/runner/configuration/values.dart
diff --git a/lib/src/runner/configuration/values.dart b/lib/src/runner/configuration/values.dart
index a266f0903c66f970008a45b7938a691827c780b7..7b992c89e3be31b21df2b6d3b7c56713cf2bd146 100644
--- a/lib/src/runner/configuration/values.dart
+++ b/lib/src/runner/configuration/values.dart
@@ -20,3 +20,8 @@ const allReporters = const ["compact", "expanded", "json"];
 final defaultReporter = inTestTests
     ? 'expanded'
     : (Platform.isWindows ? 'expanded' : 'compact');
+
+/// The default filename pattern.
+///
+/// This is stored here so that we don't have to recompile it multiple times.
+final defaultFilename = new Glob("*_test.dart");