Add a dedicated Result for skipped tests.

doc/json_reporter.md

 JSON Reporter Protocol
 ======================
 
 The test runner supports a JSON reporter which provides a machine-readable
 representation of the test runner's progress. This reporter is intended for use
 by IDEs and other tools to present a custom view of the test runner's operation
 without needing to parse output intended for humans.
 
 Note that the test runner is highly asynchronous, and users of this protocol
 shouldn't make assumptions about the ordering of events beyond what's explicitly
@@ skipping 133 matching lines @@
 ```
 
 A group event is emitted before any `TestStartEvent`s for tests in a given
 group. This is the only event that contains the full metadata about a group;
 future events will refer to the group by its opaque ID.
 
 This includes the implicit group at the root of each suite, which has a `null`
 name. However, it does *not* include implicit groups for the virtual suites
 generated to represent loading test files.
 
-The group should be considered skipped if `group.metadata.skip` is `true`. When
-a group is skipped, a single `TestStartEvent` will be emitted for a test within
-that group that will also be skipped.
+If the group is skipped, a single `TestStartEvent` will be emitted for a test
+within the group, followed by a `TestDoneEvent` marked as skipped. The
+`group.metadata.skip` field should *not* be considered authoritative for
+determining whether a group is skipped.
 
 ### TestStartEvent
 
 ```
 class TestStartEvent extends Event {
   String type = "testStart";
 
   // Metadata about the test that started.
   Test test;
 }
 ```
 
 An event emitted when a test begins running. This is the only event that
 contains the full metadata about a test; future events will refer to the test by
 its opaque ID.
 
-The test should be considered skipped if `test.metadata.skip` is `true`.
+If the test is skipped, its `TestDoneEvent` will have `skipped` set to `true`.
+The `test.metadata.skip` field should *not* be considered authoritative for
+determining whether a test is skipped.
 
 ### PrintEvent
 
 ```
 class PrintEvent extends Event {
   String type = "print";
 
   // The ID of the test that printed a message.
   int testID;
 
@@ skipping 39 matching lines @@
   String type = "testDone";
 
   // The ID of the test that completed.
   int testID;
 
   // The result of the test.
   String result;
 
   // Whether the test's result should be hidden.
   bool hidden;
+
+  // Whether the test (or some part of it) was skipped.
+  bool skipped;
 }
 ```
 
 An event emitted when a test completes. The `result` attribute indicates the
 result of the test:
 
 * `"success"` if the test had no errors.
 
 * `"failure"` if the test had a `TestFailure` but no other errors.
 
@@ skipping 150 matching lines @@
 class Metadata {
   // Whether the test case will be skipped by the test runner.
   bool skip;
 
   // The reason the test case is skipped, if the user provided it.
   String? skipReason;
 }
 ```
 
 The metadata attached to a test by a user.
+
+Note that the `skip` field should not be considered authoritative. A test may be
+skipped even if `skip` is set to `false`.