PlaceholderParser: Allow null in type context

This is so that you can do:

  parseStatement `var x: ${null};`

result in

  var x;
Add support for class member variable
Type asserts: Fix issues with TypeName and asserts

The assert functions we used for testing was wrong. It did not
handle number vs Number.

We also dropped the `moduleName` of the TypeName on the floor.
Type asserts: Fix issues with TypeName and asserts

The assert functions we used for testing was wrong. It did not
handle number vs Number.

We also dropped the `moduleName` of the TypeName on the floor.
Type asserts: Add support for symbol primitive
Generic type assertions

This adds basic support for generics for the type assertions pass.
Right now the responsibility for validating the type is part of
the test. All Traceur does is to to transform

T<x, y>

into

$traceurRuntime.genericType(T, x, y)

which now constructs a new $traceurRuntime.GenericType which the
testing assers can inspect to determine how to validate the value.
Parsing support for TypeParameters and TypeParameter
Parsing support for FunctionType
Parsing support for ConstructorType
Parse ArrayType
Parse ObjectType