experimental option to dump JS from AST

Summary:
Add a new module `AST2JS` which takes AST as input and generates
JavaScript source. It has an optional "pretty" mode where spaces,
newlines and indentation are inserted for readability. Long expressions
however are not split into multiple lines - that would require a whole
new level of complexity. The purpose here is not to implement a JS
auto-formatter, but simply to emit fairly readable code.

When "pretty" mode is disabled, no spaces and new lines are emitted (except where required).

It does not support all AST constructs yet - I had no time to add all of
them, but adding more is easy using the existing patterns. It crashes
when encountering an unsupported AST kind. It works well enough to
generate the `richards.js` benchmark.

The easiest way to implement all missing nodes is to delete
`void GenJS::visit(Node *n)`, which will cause compiler errors until
they are all added.

New command line options: `-dump-js`, `-dump-transformed-js`.

Reviewed By: avp

Differential Revision: D25295347

fbshipit-source-id: 201140c0465de87602a344d9e6a4a021238d94df

include/hermes/AST2JS/AST2JS.h

@@ -0,0 +1,28 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#ifndef HERMES_AST2JS_AST2JS_H
+#define HERMES_AST2JS_AST2JS_H
+
+namespace llvh {
+class raw_ostream;
+}
+
+namespace hermes {
+namespace ESTree {
+class Node;
+}
+
+/// Output the supplied AST as JS.
+/// \param OS the output stream
+/// \param root the root AST node
+/// \param pretty pretty print or not.
+void generateJS(llvh::raw_ostream &OS, ESTree::Node *root, bool pretty);
+
+} // namespace hermes
+
+#endif

include/hermes/Utils/Options.h

@@ -13,6 +13,8 @@ namespace hermes {
 enum OutputFormatKind {
   DumpAST,
   DumpTransformedAST,
+  DumpJS,
+  DumpTransformedJS,
   ViewCFG,
   DumpIR,
   DumpLIR,