shelldb: Make shelldb analyze also analyze the packages, since that's where most development is actually happening at the moment.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1101893002

tools/shelldb

@@ -241,6 +241,7 @@ class Analyze(object):
             "--url-mapping=dart:sky,%s" % dart_sky_path,
             "--url-mapping=dart:sky_builtin,%s" % sky_builtin_path,
             "--package-root", dev_packages_root(build_dir),
+            "--package-warnings",
             args.app_path
         ]
         return subprocess.call(analyzer_args)