Add basic support for a configuration file.

lib/src/executable.dart

Index: lib/src/executable.dart
diff --git a/lib/src/executable.dart b/lib/src/executable.dart
index fa9e2f191700e80b1fe733c0623a7284d1ccfdda..8dd546ec7bbcb36777e12f944e3bf3128ccc77af 100644
--- a/lib/src/executable.dart
+++ b/lib/src/executable.dart
@@ -7,6 +7,7 @@
 // bin.
 import 'dart:io';
 
+import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:yaml/yaml.dart';
 
@@ -84,6 +85,25 @@ main(List<String> args) async {
     return;
   }
 
+  try {
+    if (new File("dart_test.yaml").existsSync()) {
+      var fileConfiguration = new Configuration.load("dart_test.yaml");
+      configuration = fileConfiguration.merge(configuration);
+    }
+  } on SourceSpanFormatException catch (error) {
+    stderr.writeln(error.toString(color: configuration.color));
+    exitCode = exit_codes.data;
+    return;
+  } on FormatException catch (error) {
+    stderr.writeln(error.message);
+    exitCode = exit_codes.data;
+    return;
+  } on IOException catch (error) {
+    stderr.writeln(error.toString());
+    exitCode = exit_codes.noInput;
+    return;
+  }
+
   if (configuration.pubServeUrl != null && !_usesTransformer) {
     stderr.write('''
 When using --pub-serve, you must include the "test/pub_serve" transformer in