Fixes #489

This change set includes tests for most invocations of the
SetFunctionName abstract operation in the ES2015 specification.
Practical testing considerations preclude the introduction of tests for
certain invocations:

- The project is still vetting methods to sustainably test the semantics
  of the Destructuring Binding pattern across all valid productions.
  - 13.3.3.6 Runtime Semantics: IteratorBindingInitialization
  - 13.3.3.7 Runtime Semantics: KeyedBindingInitialization
- Without a loader, there is no way to access a function object declared
  in an ExportDeclaration, so `name` assignment cannot be tested in
  these cases
  - 14.1.19 Runtime Semantics: InstantiateFunctionObject
  - 14.4.12 Runtime Semantics: InstantiateFunctionObject
  - 14.5.15 Runtime Semantics: BindingClassDeclarationEvaluation
  - 15.2.3.11 Runtime Semantics: Evaluation

The `negative` frontmatter tag expresses an expectation for the behavior
of the test file as a whole. The `assert.throws` helper function offers
more fine-grained control over expectations because it may be applied to
specific statements and expressions. This makes it preferable in cases
where it may be used (i.e. when the test body does not describe a syntax
error or early error).

Re-implement assertions for errors to use the `assert.throws` helper
function wherever possible.

The `test/language/class/` directory contains a small subset of
Test262's tests for ES2015 classes. The majority of tests for classes
are organized within `test/language/statements/class/`.

- Move the tests that rely on the ClassDeclaration syntactic form from
  `test/language/class/` to `test/language/statements/class/`.
- Move the test that relies on the ClassExpression syntactic form from
  `test/language/class/` to `test/language/expressions/class/`.

- 14.5.1 Static Semantics: Early Errors
- 14.5.3 Static Semantics: ConstructorMethod
- 14.5.14 Runtime Semantics: ClassDefinitionEvaluation