Return Promise from Loader.import/module/define/loadAsScript/script.

API changes: no callback args.

To update code, replace any callback arguments with .then(callback, errback):
  loader.import(name, {}, callback, errback);
become
  loader.import(name, {}).then(callback, errback);

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/747

Closes #747.
Reorganize InternalLoader (as driver) and LoaderHooks (as services).

Move semantics/ModuleAnalyzer and /ModuleSymbol to codegeneration/module
Rename ModuleAnalyzer to ExportListBuilder.
Rework calls to ExportListBuilder to separate codeUnit from trees.
Move LoaderHooks.transformDependencies to Loader.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/748

Closes #748.
Fix issue with labelled blocks in generators

Labelled blocks, needs to be handled similar to the loops where we have to transform the break statements.

Fixes #605

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/750

Closes #750, closes #605.
Test Promise reject path for Loader.import/define/module/script/load.

Fix .script(), it was not rejecting.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/751

Closes #751.
Refactor InternalLoader to use promises

Refactor InternalLoader to use promises instead of ad-hoc listener interface.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/756

Closes #756.
Implement --script command line option to parse as Script.

Option --script followed by name of a file to be parsed as Script.
Default changed: now files are parsed as Module.
Test changes to verify values via globlals (modules have no value).
Module compilation simulates System.import() but post-pending
 System.get(normalizedName) to the output.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/760

Closes #760.
Update version to 0.0.21

I should have updated the semver in the previous commit. Now I will have to do it twice.

TBR=@arv

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/761

Closes #761.
Propagate --script to first self-hosting build.

Update semver to 0.0.22

TBR=@arv

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/762

Closes #762.
Verify that Object.getPrototypeOf(module) is null

Fixes #763

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/767

Closes #767, closes #763.
export DirectExportVisitor

Here's the pull request for #769.

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/772

Closes #772.
Fix alphabetical ordering from last commit
Merge branch 'master' into repl-love
Cleanup