As originally implemented, a SingleNameBinding within a BindingPattern
was incorrectly interpreted as an assignment if an initializer was
present and that initializer was itself an AssignmentExpresion.
For example:

    let x;
    { let [x = y = 1] = []; }
    print(x); // expected: undefined, actual: 1

Extend the heuristic that detects the "context" of a destructuring
pattern to account for AssignmentExpressions within SingleNameBindings.

BUG=v8:4891
LOG=N
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/1859423002

Cr-Commit-Position: refs/heads/master@{#35334}