Add basic support for a configuration file.

lib/src/executable.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(nweiz): This is under lib so that it can be used by the unittest dummy
 // package. Once that package is no longer being updated, move this back into
 // bin.
 import 'dart:io';
 
+import 'package:source_span/source_span.dart';
 import 'package:stack_trace/stack_trace.dart';
 import 'package:yaml/yaml.dart';
 
 import 'runner.dart';
 import 'runner/application_exception.dart';
 import 'runner/configuration.dart';
 import 'runner/version.dart';
 import 'util/exit_codes.dart' as exit_codes;
 import 'util/io.dart';
 import 'utils.dart';
@@ skipping 57 matching lines @@
     var version = testVersion;
     if (version == null) {
       stderr.writeln("Couldn't find version number.");
       exitCode = exit_codes.data;
     } else {
       print(version);
     }
     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
 your pubspec:
 
 transformers:
 - test/pub_serve:
     \$include: test/**_test.dart
 ''');
     exitCode = exit_codes.data;
@@ skipping 52 matching lines @@
     output = stderr;
   }
 
   output.write("""$message
 
 Usage: pub run test:test [files or directories...]
 
 ${Configuration.usage}
 """);
 }