Fix analyzer warnings for browser_controller.
Fix analyzer warnings for browser_controller.

BUG=
R=terry@google.com

Review URL: https://codereview.chromium.org/2069243002 .

dart-lang/sdk,dartino/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dartino/dart-sdk,dartino/dart-sdk,dart-archive/dart-sdk,dart-archive/dart-sdk,dart-lang/sdk,dart-lang/sdk