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 transform

T<x, y>

into

$traceurRuntime.genericType(T, x, y)

which now constructs a new $traceurRuntime.GenericType which the
testing asserts can inspect to determine how to validate the value.
Support sourcemaps in node execution.

Add dev-dependency on source-map-support.
Add --sourcemaps=memory option to store sourcemap in memory (actually
to not store it as data URL or file on disk.
When sourcemap option selected, supply sourcemap out of memory cache.
Replace Loader.getSourceMapInfo() based on codeUnit lookup with
Loader.getSourceMap(outputName).

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1475

Closes #1475.
Remove unused code from PR #1475

TBR=@arv

Review URL: https://api.github.com/repos/google/traceur-compiler/issues/1479

Closes #1479.
Allow >> in type generics syntax

We support this in a similar way to the way parsing regexps work.
When we look for the `>` we let the scanner treat `>>` as `>` and
decrease the position of the scanner so that next token it sees
starts after the first `>`.

Fixes #1467
Allow .js at the end of module names

Issue #1221
0.0.74
Commit binaries for 0.0.75