Note that this also modifies mjsunit.js to allow the {failWithMessage} method to be monkey-patched by a test. This is necessary because assertions which fail in a promise's then-clause would normally only throw an exception, which is swallowed by the promise, causing the test to silently pass. Instead, patching this {failWithMessage} functionality allows then clauses to use the full assertion machinery of mjsunit.js.

R=ulan@chromium.org, gsathya@chromium.org

BUG=

Review-Url: https://codereview.chromium.org/2752043002
Cr-Commit-Position: refs/heads/master@{#43875}