Fix NoErrorsGenerator failure by fixing test flag settings.

The test assumed that 'class' was allowed in sloppy mode,
but that's only true with the --harmony-sloppy flag in the 5.1 branch.
This patch simply flips that flag to true in the failing test.

BUG=v8:4974
LOG=N
TBR=littledan@chromium.org

Review URL: https://codereview.chromium.org/2004563002 .

Cr-Commit-Position: refs/branch-heads/5.1@{#53}
Cr-Branched-From: 167dc63b4c9a1d0f0fe1b19af93644ac9a561e83-refs/heads/5.1.281@{#1}
Cr-Branched-From: 03953f52bd4a184983a551927c406be6489ef89b-refs/heads/master@{#35282}