Fix broken #platform-selector-syntax and #platform-specific-configuration links (#963)

Also adjust the platform-specific-configuration links.

pkgs/test/doc/configuration.md

@@ -557,7 +557,7 @@ cares about the platform the *test* is running on (like the
 [`@OnPlatform` annotation][@OnPlatform]). It sets configuration for particular
 tests that are running on matching platforms.
 
-[@OnPlatform]: https://github.com/dart-lang/test/blob/master/README.md#platform-specific-configuration
+[@OnPlatform]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-specific-configuration
 
 ### `on_os`
 

pkgs/test_api/lib/src/frontend/on_platform.dart

@@ -6,7 +6,7 @@
 ///
 /// See [the README][onPlatform].
 ///
-/// [onPlatform]: https://github.com/dart-lang/test/blob/master/README.md#platform-specific-configuration
+/// [onPlatform]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-specific-configuration
 class OnPlatform {
   final Map<String, dynamic> annotationsByPlatform;
 

pkgs/test_api/lib/src/frontend/test_on.dart

@@ -6,7 +6,7 @@
 ///
 /// For the full syntax of [expression], see [the README][].
 ///
-/// [the README]: https://github.com/dart-lang/test/#platform-selector-syntax
+/// [the README]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-selectors
 class TestOn {
   /// The expression specifying the platform.
   final String expression;

pkgs/test_api/lib/test_api.dart

@@ -43,7 +43,7 @@ Declarer get _declarer => Zone.current[#test.declarer] as Declarer;
 /// [group]s. If [testOn] is passed, it's parsed as a [platform selector][]; the
 /// test will only be run on matching platforms.
 ///
-/// [platform selector]: https://github.com/dart-lang/test/#platform-selector-syntax
+/// [platform selector]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-selectors
 ///
 /// If [timeout] is passed, it's used to modify or replace the default timeout
 /// of 30 seconds. Timeout modifications take precedence in suite-group-test
@@ -123,7 +123,7 @@ void test(description, body(),
 /// If [testOn] is passed, it's parsed as a [platform selector][]; the test will
 /// only be run on matching platforms.
 ///
-/// [platform selector]: https://github.com/dart-lang/test/#platform-selector-syntax
+/// [platform selector]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-selectors
 ///
 /// If [timeout] is passed, it's used to modify or replace the default timeout
 /// of 30 seconds. Timeout modifications take precedence in suite-group-test

pkgs/test_core/lib/test_core.dart

@@ -78,7 +78,7 @@ Declarer get _declarer {
 /// [group]s. If [testOn] is passed, it's parsed as a [platform selector][]; the
 /// test will only be run on matching platforms.
 ///
-/// [platform selector]: https://github.com/dart-lang/test/#platform-selector-syntax
+/// [platform selector]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-selectors
 ///
 /// If [timeout] is passed, it's used to modify or replace the default timeout
 /// of 30 seconds. Timeout modifications take precedence in suite-group-test
@@ -158,7 +158,7 @@ void test(description, body(),
 /// If [testOn] is passed, it's parsed as a [platform selector][]; the test will
 /// only be run on matching platforms.
 ///
-/// [platform selector]: https://github.com/dart-lang/test/#platform-selector-syntax
+/// [platform selector]: https://github.com/dart-lang/test/tree/master/pkgs/test#platform-selectors
 ///
 /// If [timeout] is passed, it's used to modify or replace the default timeout
 /// of 30 seconds. Timeout modifications take precedence in suite-group-test