pkg/docgen: fix type warnings

pkg/pkg.status

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Don't run any test-like files that show up in packages directories. It
 # shouldn't be necessary to run "pub install" in these packages, but if you do
 # it shouldn't break the tests.
 */packages/*/*: Skip
 */*/packages/*/*: Skip
 */*/*/packages/*/*: Skip
@@ skipping 141 matching lines @@
 # These tests are runtime negative but statically positive, so we skip
 # them in the analyzer.
 mock/test/mock_regexp_negative_test: Skip
 mock/test/mock_stepwise_negative_test: Skip
 polymer/example/canonicalization: Skip
 polymer/example/canonicalization2: Skip
 polymer/example/canonicalization3: Skip
 
 third_party/angular_tests/browser_test: StaticWarning # Issue 15890
 
-docgen/test/multi_library_test: StaticWarning # Issue 18232
-docgen/test/generate_json_test: StaticWarning # Issue 18232
-docgen/test/single_library_test: StaticWarning # Issue 18232
-docgen/test/only_lib_content_in_pkg: StaticWarning # Issue 18232
-docgen/test/only_lib_content_in_pkg_test: StaticWarning # Issue 18232
-docgen/test/typedef_test: StaticWarning # Issue 18232
-docgen/test/inherited_comments_test: StaticWarning # Issue 18232
-docgen/test/constant_argument_test: StaticWarning # Issue 18232
-
 [ $compiler == dart2js && $runtime == none]
 polymer/example/canonicalization: Skip
 polymer/example/canonicalization2: Skip
 polymer/example/canonicalization3: Skip
 
 [ $compiler == dart2js && $csp ]
 matcher/test/mirror_matchers_test: Skip # Issue 12151
 polymer/test/noscript_test: Fail # Issue 17326
 polymer/test/js_interop_test: Fail # Issue 17326
 
@@ skipping 174 matching lines @@
 third_party/angular_tests/browser_test/core/scope: Skip # Requires checked mode.
 third_party/angular_tests/browser_test/core/zone: Skip # Requires checked mode.
 third_party/angular_tests/browser_test/directive/ng_repeat: Skip # Issue 17152
 third_party/angular_tests/browser_test/change_detection/dirty_checking_change_detector: Skip # Issue 17152
 
 [ $use_repository_packages ]
 analyzer/test/*: PubGetError
 
 [ $runtime == ie9 ]
 polymer: Pass, Fail, Timeout # Issue 16717