Support tag configuration.

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]
+
+tags:
+  # Integration tests are even slower, so increase the timeout again.
+  integration: {timeout: 2x}
+
+  # Sanity tests are quick and verify that nothing is obviously wrong. Declaring
+  # the tag allows us to tag tests with it without getting warnings.
+  sanity:
 ```
 
-* [Config Fields](#config-fields)
+* [Test Configuration](#test-configuration)
+  * [`timeout`](#timeout)
+  * [`verbose_trace`](#verbose_trace)
+  * [`js_trace`](#js_trace)
+* [Runner Configuration](#runner-configuration)
   * [`paths`](#paths)
   * [`filename`](#filename)
   * [`platforms`](#platforms)
   * [`concurrency`](#concurrency)
   * [`pub_serve`](#pub_serve)
-  * [`timeout`](#timeout)
   * [`reporter`](#reporter)
-  * [`verbose_trace`](#verbose_trace)
-  * [`js_trace`](#js_trace)
+* [Configuring Tags](#configuring-tags)
 
-## Config Fields
+## Test Configuration
 
-These fields directly affect the behavior of the test runner. They go at the
-root of the configuration file.
+There are two major categories of configuration field: "test" and "runner". Test
+configuration controls how individual tests run, while
+[runner configuration](#runner-configuration) controls the test runner as a
+whole. Both types of fields may be used at the top level of a configuration
+file. However, because different tests can have different test configuration,
+only test configuration fields may be used to
+[configure tags](#configuring-tags).
+
+### `timeout`
+
+This field indicates how much time the test runner should allow a test to remain
+inactive before it considers that test to have failed. It has three possible
+formats:
+
+* The string "none" indicates that tests should never time out.
+
+* A number followed by a unit abbreviation indicates an exact time. For example,
+  "1m" means a timeout of one minute, and "30s" means a timeout of thirty
+  seconds. Multiple numbers can be combined, as in "1m 30s".
+
+* A number followed by "x" indicates a multiple. This is applied to the default
+  value of 30s.
+
+```yaml
+timeout: 1m
+```
+
+### `verbose_trace`
+
+This boolean field controls whether or not stack traces caused by errors are
+trimmed to remove internal stack frames. This includes frames from the Dart core
+libraries, the [`stack_trace`][stack_trace] package, and the `test` package
+itself. It defaults to `false`.
+
+[stack_trace]: https://pub.dartlang.org/packages/stack_trace
+
+```yaml
+verbose_trace: true
+```
+
+### `js_trace`
+
+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
+```
+
+## Runner Configuration
+
+Unlike [test configuration](#test-configuration), runner configuration affects
+the test runner as a whole rather than individual tests. It can only be used at
+the top level 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]
@@ skipping 52 matching lines @@
 
 This field indicates that the test runner should run against a `pub serve`
 instance by default, and provides the port number for that instance. Note that
 if there is no `pub serve` instance running at that port, running the tests will
 fail by default.
 
 ```yaml
 pub_serve: 8081
 ```
 
-### `timeout`
-
-This field indicates how much time the test runner should allow a test to remain
-inactive before it considers that test to have failed. It has three possible
-formats:
-
-* The string "none" indicates that tests should never time out.
-
-* A number followed by a unit abbreviation indicates an exact time. For example,
-  "1m" means a timeout of one minute, and "30s" means a timeout of thirty
-  seconds. Multiple numbers can be combined, as in "1m 30s".
-
-* A number followed by "x" indicates a multiple. This is applied to the default
-  value of 30s.
-
-```yaml
-timeout: 1m
-```
-
 ### `reporter`
 
 This field indicates the default reporter to use. It may be set to "compact",
 "expanded", or "json" (although why anyone would want to default to JSON is
 beyond me). It defaults to "expanded" on Windows and "compact" everywhere else.
 
 ```yaml
 reporter: expanded
 ```
 
-### `verbose_trace`
+## Configuring Tags
 
-This boolean field controls whether or not stack traces caused by errors are
-trimmed to remove internal stack frames. This includes frames from the Dart core
-libraries, the [`stack_trace`][stack_trace] package, and the `test` package
-itself. It defaults to `false`.
+The `tag` field can be used to apply [test configuration](#test-configuration)
+to all tests [with a given tag][tagging tests]. It takes a map from tag names to
+configuration maps. These configuration maps are just like the top level of the
+configuration file, except that they may not contain
+[runner configuration](#runner-configuration).
 
-[stack_trace]: https://pub.dartlang.org/packages/stack_trace
+[tagging tests]: https://github.com/dart-lang/test/blob/master/README.md#tagging-tests
 
 ```yaml
-verbose_trace: true
+tags:
+  # Integration tests need more time to run.
+  integration:
+    timeout: 1m
 ```
 
-### `js_trace`
-
-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`.
+Tags may also have no configuration associated with them. The test runner prints
+a warning whenever it encounters a tag it doesn't recognize, and this the best
+way to tell it that a tag exists.
 
 ```yaml
-js_trace: true
+# We occasionally want to use --tags or --exclude-tags on these tags.
+tags:
+  # A test that spawns a browser.
+  browser:
+
+  # A test that needs Ruby installed.
+  ruby:
 ```
+
+Tag configuration is applied at whatever level the tag appears—so if a group is
+tagged as `integration`, its timeout will take precedence over the suite's
+timeout but not any tests'. If the group itself had a timeout declared, the
+group's explicit timeout would take precedence over the tag.
+
+If multiple tags appear at the same level, and they have conflicting
+configurations, the test runner *does not guarantee* what order they'll be
+resolved in. In practice, conflicting configuration is pretty unlikely and it's
+easy to just explicitly specify what you want on the test itself.