* Check to make sure default npm exists at path before trying to use it
**Bug**
If the typings installer is run under a copy of node that does not include npm—but on a machine that does have npm installed—it will incorrectly try to use the npm that does not exist next to its running version of node
**Fix**
Make sure that we check that npm we select exists before trying to use it as the default. Otherwise, fall back to using plain old `npm`
* Add command line flag to gate new behavior
* Fix missing semicolon
* Rename JS concepts
1. Assignment declaration -- an assignment that is treated like a
declaration. Previously called [JS] special (assignment|declaration),
among other things.
2. Expando -- a value that can be used as a target in assignment
declarations. Currently, a class, function or empty object literal.
Functions are allowed in Typescript, too. Previously called a JS
container, JS initializer or expando object.
3. JavaScript -> Javascript. This is annoying to type, and looks like
'Java Script' in a camelCase world.
Everything is a pure rename as far as I know. The only test change is
the API baselines, which reflect the rename from SymbolFlags.JSContainer
to SymbolFlags.Assignment.
* Remove TODO
* Rename Javascript->JS
Note that this introduces a variable name collision in a couple of
places, which I resolved like this:
```ts
const isInJavascript = isInJSFile(node);
```
* Optimize sourcemap application more
* Remove test-only memory hog sourceMapDecodedMappings field
* Update for style, remove unused function that triggers warnings in node 10
* Avoid all raw buffer constructor calls
* Small TDZ fix
