Reland JSON modules by reverting a530f6f,
resolving merge conflicts, and updating the change with respect to the
string changes in a9ef15d.

Update all [[RequestedModules]] references to reflect the change from string to ModuleRequest.
Pass ModuleRequest instead of url to 'internal module graph fetching procedure'.
Add optional ModuleRequest param to 'fetch a single module script'.
Add checks in 'fetch a single module script' to fail if the type doesn't match.

…module script graph', HostResolveImportedModule, and HostResolveImportedModuleDynamically.

… type is valid but doesn't match the requested type.

…pecify type at the point of preload. Achieve this by adding 'module type must match' flag to 'fetch a single module script', which is unset only in the case of modulepreload.

…b1078ae

…type.

…oposal spec

…'; I'll add this in a separate PR.

…defining steps in HTML spec

…does not match the type specified at the import site.

…equest matches its type and it is evalutated

…ype, cache the fetch response instead of creating and caching the module script. The module script is created only when there is a type match. This addreses concerns that parsing modules (perhaps even types of modules that don't yet exist) could have side-effects.

… the Travis CI build doesn't like.

…efully unstick CI build, which seems to be stuck.

Resolve many merge conflicts in /source.

…ensure they are synchronous with respect to the calling 'fetch a single module script'

…gnized import attributes, and assert that there is only one of key 'type' (which should be guaranteed by Ecma262)

…t with matching type', retrieve it from the module map within the algorithm.

…nning in parallel

…tions

… to 'Import Assertions'

…sponse body in the case of a failed type check.

…attributes

…tch a single module script'.

…ences back to just 'module script'

…attributes-no-json

…n key, and that in practice fetches and module evalutations will only occur once per specifier

… for unrecognized types (which is to say, any type at all).

…attributes-no-json

…tion, use 'javascript' instead of undefined, fix indents.

…pe, misc whitespace issues.

…attributes-no-json

…y evaluate

…pec recommendation is not a sure thing.

…tributes-no-json