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

A number of Array.prototype.find tests were missing a feature specification.

One of the Array.prototype.fill tests specifies an es5id. Changed to an es6id.

Use the more common array syntax for includes

Fix monkey yaml's handling of carriage return

monkeyYaml didn't split lines correctly leading to \r in resulting
values.

Fixes #295

Insert omitted `negative` tag

Remove `$FAIL` function

This test exercises an early error, so it should be declared with the
`negative` tag.