dart2js: Fix types in js_parser_statements_test.

Review URL: https://codereview.chromium.org//697873005

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41604 260f80e4-7a28-3924-810f-c04153c831b5
dart2js: Use type-checks in later operations.

We correctly GVNed type-checks (in checked mode), but we didn't always use the checks in later operations.

R=herhut@google.com

Review URL: https://codereview.chromium.org//699593004

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@41605 260f80e4-7a28-3924-810f-c04153c831b5