In ECMAScript 5, assignment to a non-reference value throws a runtime
ReferenceError. ECMAscript 6 specifies an early ReferenceError in these
cases. Tests for this behavior have been authored to pass in both cases.
Simplify these tests to describe and assert the early error.

Complete migration of website assets

Update the URLs used by dynamic script loading code according to the new
location of the recently-moved web site scripts.

Remove unnecessary "include" file

Move web scripts

Remove empty "include" file

Some JavaScript source files are only relevant in the context of the
Test262 website. They should not be explicitly included by individual
tests, so their presence in the `harness/` directory alongside "include"
files is misleading.

Move the scripts to a location within the `website/` directory to
better-reflect their intended use. Update the relevant HTML templates
with the new locations.

Although test files once expressed dependencies on external files using
a global `$INCLUDE` function, that pattern was removed in favor of
declarative meta-data [1].

Remove the associated logic from the Python runner and the browser.

[1] See commit d4354d14.

The `fnExists` function defines a generic way to determine if any number
of values are function instances. Because it is only used by a single
test, the additional complexity required by the generalized code (and
the organizational drawbacks to maintaining another "include" file) are
not justified. Remove the file and update the test to assert the
function's existence directly.

The `environment.js` file has been empty since its initial introduction
to this project [1]. It has no effect on any of the contexts in which it
is currently referenced, so it may be safely removed.

[1] be82787a

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