Add configuration support for choosing test paths.

doc/package_config.md

 Each package may include a configuration file that applies to the package as a
 whole. This file can be used to provide custom defaults for various options, to
 define configuration for multiple files, and more.
 
 The file is named `dart_test.yaml` and lives at the root of the package, next to
 the package's pubspec. Like the pubspec, it's a [YAML][] file. Here's an
 example:
 
 [YAML]: http://yaml.org/
 
 ```yaml
 # This package's tests are very slow. Double the default timeout.
 timeout: 2x
 
 # This is a browser-only package, so test on content shell by default.
 platforms: [content-shell]
 ```
 
 * [Config Fields](#config-fields)
 
 ## Config Fields
 
 These fields directly affect the behavior of the test runner. They go at the
 root of the configuration file.
 
+### `paths`
+
+This field indicates the default paths that the test runner should run. These
+paths are usually directories, although single filenames may be used as well.
+Paths must be relative, and they must be in URL format so that they're
+compatible across operating systems. This defaults to `[test]`.
+
+```yaml
+paths: [dart/test]
+
+paths:
+- test/instantaneous
+- test/fast
+- test/middling
+```
+
+### `filename`
+
+This field indicates the filename pattern that the test runner uses to find test
+files in directories. All files in directories passed on the command line (or in
+directories in [`paths`](#paths), if none are passed) whose basenames match this
+pattern will be loaded and run as tests.
+
+This supports the full [glob syntax][]. However, since it's only compared
+against a path's basename, path separators aren't especially useful. It defaults
+to `"*_test.dart"`.
+
+```yaml
+filename: "test_*.dart"
+```
+
+[glob syntax]: https://github.com/dart-lang/glob#syntax
+
 ### `platforms`
 
 This field indicates which platforms tests should run on by default. It allows
-the same paltform identifiers that can be passed to `--platform`. If multiple
+the same platform identifiers that can be passed to `--platform`. If multiple
 platforms are included, the test runner will default to running tests on all of
 them. This defaults to `[vm]`.
 
 ```yaml
 platforms: [content_shell]
 
 platforms:
 - chrome
 - firefox
 ```
@@ skipping 67 matching lines @@
 
 This boolean field controls whether or not stack traces caused by errors that
 occur while running Dart compiled to JS are converted back to Dart style. This
 conversion uses the source map generated by `dart2js` to approximate the
 original Dart line, column, and in some cases member name for each stack frame.
 It defaults to `false`.
 
 ```yaml
 js_trace: true
 ```