Add location information to the JSON reporter.

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 272 matching lines @@
 
   // The ID of the suite containing this test.
   int suiteID;
 
   // The IDs of groups containing this test, in order from outermost to
   // innermost.
   List<int> groupIDs;
 
   // The test's metadata, including metadata from any containing groups.
   Metadata metadata;
+
+  // The (1-based) line on which the test was defined, or `null`.
+  int line;
+
+  // The (1-based) column on which the test was defined, or `null`.
+  int column;
+
+  // The URL for the file in which the test was defined, or `null`.
+  String url;
 }
 ```
 
 A single test case. The test's ID is unique in the context of this test run.
 It's used elsewhere in the protocol to refer to this test without including its
 full representation.
 
 Most tests will have at least one group ID, representing the implicit root
 group. However, some may not; these should be treated as having no group
 metadata.
 
+The `line`, `column`, and `url` fields indicate the location the `test()`
+function was called to create this test. They're treated as a unit: they'll
+either all be `null` or they'll all be non-`null`. The URL is always absolute,
+and may be a `package:` URL.
+
 ### Suite
 
 ```
 class Suite {
   // An opaque ID for the group.
   int id;
 
   // The platform on which the suite is running.
   String? platform;
 
@@ skipping 28 matching lines @@
   int suiteID;
 
   // The ID of the group's parent group, unless it's the root group.
   int? parentID;
 
   // The group's metadata, including metadata from any containing groups.
   Metadata metadata;
 
   // The number of tests (recursively) within this group.
   int testCount;
+
+  // The (1-based) line on which the group was defined, or `null`.
+  int line;
+
+  // The (1-based) column on which the group was defined, or `null`.
+  int column;
+
+  // The URL for the file in which the group was defined, or `null`.
+  String url;
 }
 ```
 
 A group containing test cases. The group's ID is unique in the context of this
 test run. It's used elsewhere in the protocol to refer to this group without
 including its full representation.
 
 The implicit group at the root of each test suite has `null` `name` and
 `parentID` attributes.
 
+The `line`, `column`, and `url` fields indicate the location the `group()`
+function was called to create this group. They're treated as a unit: they'll
+either all be `null` or they'll all be non-`null`. The URL is always absolute,
+and may be a `package:` URL.
+
 ### Metadata
 
 ```
 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.