Add tests for Object.setPrototypeOf

Update browser runner to honor `onlyStrict` flag

Add tests for `for..of` iteration over built-ins

Fix issue with math_precision.js

It was missing vars which was causing issues with our test harness
since we run all the files in strict mode.

Update spec references to use canonical source

Extend coverage for Object.assign

Add tests for WeakSet

Fix bugs in Promise tests

This test's description concerns the behavior of `Promise.all` when the
IteratorStep abstract operation fails due to an abrupt completion
returned by the iterator's `next` method. The test body did not actually
assert that functionality.

Update the test body to correctly define the requisite iterator and
assert that the specific error created is the one thrown from the
invocation of `Promise.all`

Some tests specifically concern the application of the `use strict`
directive as it appears in JavaScript source code. These tests should
*not* be run with the `onlyStrict` flag because relying on the test
runner to enable strict mode makes the semantics of the source code
irrelevant. Update these tests to use the `noStrict` flag.

Other tests concern language semantics that are only valid in strict
mode, but the mechanism for enabling strictness is inconseqential.
Update these tests to use the `onlyStrict` flag and remove any redundant
`use strict` directive prologues contained within.

Still other tests are valid both within and outside of strict mode.
In keeping with the majority of other tests, do not specify any
restrictions on the environments in which these tests may be run.

Unlike the console runner, the browser runner does not modify the
strictness of tests prior to running them. Regardless of a given test's
metadata, it runs every test exactly once, and it never enables strict
mode. This means that tests intended to function in strict mode must
declare the "use strict"; directive prologue in addition to the
`onlyStrict` flag.

For any test that specifies the `onlyStrict` metadata flag, transform
the source code by injecting a "use strict" directive prologue prior to
running the test.

Add tests for Object-Assign

Update browser runner to inject `assert.js`

Since the Python runner was updated to include `assert.js` in all tests
unconditionally, a number of tests have been written that implicitly
rely on its presence. The browser runner does not currently provide this
file's contents to these tests, so they fail unconditionally.

Update the browser runner to inject that file's contents into every test
context.

Note: the existing approach to file retrieval (namely loading via
synchronous XHR requests) is inefficient and deprecated in some
browsers. It is honored here for the sake of consistency and to minimize
the changeset necessary to fix the browser runner.

Add tests for MethodDefinition forms new to ES6

Remove $PRINT helper function

Fix typo in test meta-data

The `Negative` tag accepts a string value (not a list)

The `$PRINT` helper function has no effect on test behavior. The
following tests use it to report assertion outcomes (and as a result
will fail silently):

- test/language/function-code/S10.2.1_A4_T2.js
- test/language/function-code/S10.2.1_A5.1_T1.js
- test/language/function-code/S10.2.1_A5.1_T2.js
- test/language/function-code/S10.2.1_A5.2_T1.js

Remove the function definition and all references within tests. Update
tests that use it as an error reporting mechanism to instead use an
appropriate `assert` helper function.

The Array.prototype.fill function is an ES6 function.

The Array.prototype.find function is an ES6 function.

ES6 numeric literals

Fix tautological pattern in tests

In neglecting to assert the type of error thrown (or that any error was
thrown at all), these tests cannot fail. Refactor the tests to use the
`assert.throws` helper method, which takes these details into
consideration.

Add tests for Proxy