add --x-package-warnings-prefix option to dartanalyzer

pkg/analyzer_cli/test/package_prefix_test.dart

+import 'dart:io' show exitCode;
+
+import 'package:analyzer_cli/src/driver.dart' show Driver, outSink, errorSink;
+import 'package:analyzer_cli/src/options.dart' show ExitHandler, exitHandler;
+import 'package:unittest/unittest.dart';
+
+main() {
+  group('--x-package-warnings-prefix', () {
+    _Runner runner;
+
+    setUp(() {
+      runner = new _Runner.setUp();
+    });
+
+    tearDown(() {
+      runner.tearDown();
+      runner = null;
+    });
+
+    test('shows only the hint whose package matches the prefix', () {
+      runner.run([
+        "--packages",
+        "test/data/package_prefix/packagelist",
+        "--x-package-warnings-prefix=f",
+        "test/data/package_prefix/main.dart"
+      ]);
+      expect(runner.stdout, contains('1 hint found'));
+      expect(runner.stdout, contains('Unused import'));
+      expect(runner.stdout, contains('package_prefix/pkg/foo/foo.dart'));
+      expect(runner.stdout, isNot(contains('bar.dart')));
+    });
+  });
+}
+
+class _Runner {
+  final _stdout = new StringBuffer();
+  final _stderr = new StringBuffer();
+
+  final StringSink _savedOutSink;
+  final StringSink _savedErrorSink;
+  final int _savedExitCode;
+  final ExitHandler _savedExitHandler;
+
+  _Runner.setUp()
+      : _savedOutSink = outSink,
+        _savedErrorSink = errorSink,
+        _savedExitHandler = exitHandler,
+        _savedExitCode = exitCode {
+    outSink = _stdout;
+    errorSink = _stderr;
+  }
+
+  void run(List<String> args) {
+    new Driver().start(args);
+    if (stderr.isNotEmpty) {
+      fail("Unexpected output to stderr:\n$stderr");
+    }
+  }
+
+  String get stdout => _stdout.toString();
+  String get stderr => _stderr.toString();
+
+  void tearDown() {
+    outSink = _savedOutSink;
+    errorSink = _savedErrorSink;
+    exitCode = _savedExitCode;
+    exitHandler = _savedExitHandler;
+  }
+}