Add a Pub test that runs against the real Dart SDK.

utils/tests/pub/install/sdk/check_out_test.dart

Index: utils/tests/pub/install/sdk/check_out_test.dart
diff --git a/utils/tests/pub/install/sdk/check_out_test.dart b/utils/tests/pub/install/sdk/check_out_test.dart
index 5c07b200681e6bdcfc576e3c490b32e4e8313873..c5ae131cc745612d0ccae52cd192ef6a07d4aa59 100644
--- a/utils/tests/pub/install/sdk/check_out_test.dart
+++ b/utils/tests/pub/install/sdk/check_out_test.dart
@@ -32,4 +32,27 @@ main() {
 
     run();
   });
+
+  // This is necessary to test that the "revision" file in the SDK remains in a
+  // format that Pub understands.
+  test('checks out a package from the real SDK', () {
+    ensureBuild();
+
+    dir(appPath, [
+      pubspec({
+        "name": "myapp",
+        "dependencies": {"unittest": {"sdk": "unittest"}}
+      })
+    ]).scheduleCreate();
+
+    schedulePub(args: ['install'],
+        output: const RegExp(r"Dependencies installed!$"),
+        realSdk: true);
+
+    dir(packagesPath, [
+      dir("unittest")
+    ]);
+
+    run();
+  });
 }