The spec says we should GetV the "pull" and "cancel" properties of the underlyingSource object only once, under the ReadableStream constructor, and the constructor should throw if either value isn't either callable or undefined. https://streams.spec.whatwg.org/#rs-constructor -> https://streams.spec.whatwg.org/#set-up-readable-stream-default-controller-from-underlying-source -> https://streams.spec.whatwg.org/#create-algorithm-from-underlying-method Currently we do none of this in the constructor, and we Get these properties like any other JS method call, each time we need to call them. This causes test failures in several wpt-tests: testing/web-platform/tests/streams/readable-streams/ bad-underlying-sources*.html constructor*.html general*.html

Pushed by jorendorff@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/345ad3e746e8 Fix ReadableStream constructor handling of "pull" and "cancel" methods. r=arai

Backed out 5 changesets (bug 1507950, bug 1503012, bug 1507943, bug 1512050, bug 1512008) for spidermonkey and jit failures Log: https://treeherder.mozilla.org/logviewer.html#/jobs?job_id=215990151&repo=autoland&lineNumber=90481 Backout: https://hg.mozilla.org/integration/autoland/rev/7eb42458e2d82b085a298cb3b7801cab2a3d4c51

Pushed by jorendorff@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/efcada818361 Fix ReadableStream constructor handling of "pull" and "cancel" methods. r=arai