Enable flutter plugin test

.travis.yml

@@ -4,9 +4,9 @@ dart:
   - stable
   - "dev/raw/latest"
 env:
+  - DARTDOC_BOT=flutter
   - DARTDOC_BOT=sdk-analyzer
   - DARTDOC_BOT=main
-  - DARTDOC_BOT=flutter
   - DARTDOC_BOT=packages
   - DARTDOC_BOT=sdk-docs
 script: ./tool/travis.sh

test/compare_output_test.dart

@@ -67,9 +67,7 @@ void main() {
               'Top level package requires Flutter but FLUTTER_ROOT environment variable not set|test_package_flutter_plugin requires the Flutter SDK, version solving failed')));
       expect(result.stderr, isNot(contains('asynchronous gap')));
       expect(result.exitCode, isNot(0));
-    },
-        skip:
-            true /* TODO(gspencer): Re-enable as soon as Flutter's config is sane again. */);
+    });
 
     test("Validate --version works", () async {
       var args = <String>[dartdocBin, '--version'];

testing/test_package_flutter_plugin/.gitignore

@@ -0,0 +1 @@
+android/**

tool/grind.dart

@@ -592,8 +592,7 @@ Future<void> serveFlutterDocs() async {
 }
 
 @Task('Validate flutter docs')
-// TODO(jcollins-g): add buildDartdocFlutterPluginDocs once passing
-@Depends(buildFlutterDocs)
+@Depends(buildFlutterDocs, buildDartdocFlutterPluginDocs)
 void validateFlutterDocs() {}
 
 @Task('Build flutter docs')
@@ -641,6 +640,11 @@ class FlutterRepo {
       ['precache'],
       workingDirectory: flutterPath,
     );
+    await launcher.runStreamed(
+      bin,
+      ['update-packages'],
+      workingDirectory: flutterPath,
+    );
   }
 
   factory FlutterRepo.fromPath(String flutterPath, Map<String, String> env,
@@ -924,6 +928,8 @@ Future<WarningsCollection> _buildDartdocFlutterPluginDocs() async {
           [
             '--enable-asserts',
             pathLib.join(Directory.current.path, 'bin', 'dartdoc.dart'),
+            '--exclude-packages',
+            'Dart', // TODO(jcollins-g): dart-lang/dartdoc#1431
             '--json',
             '--link-to-remote',
             '--output',