[CS2] Don’t require async/await support to run coffee (#4679)

* Get `coffee` command working again in Node 6, by converting the ‘await’ wrapper in the REPL to use a Promise instead of the ‘await’ keyword; add tests for REPL ‘await’ wrapper, including test to skip async tests if the runtime doesn’t support them

* Code review

* Let's support Node 6+ if we can

lib/coffeescript/repl.js

@@ -26,7 +26,7 @@
       }
     })(),
     historyMaxInputSize: 10240,
-    eval: async function(input, context, filename, cb) {
+    eval: function(input, context, filename, cb) {
       var Assign, Block, Call, Code, Literal, Value, ast, err, isAsync, js, referencedVars, result, token, tokens;
       // XXX: multiline hack.
       input = input.replace(/\uFF00/g, '\n');
@@ -71,10 +71,11 @@
         result = runInContext(js, context, filename);
         // Await an async result, if necessary
         if (isAsync) {
-          result = (await result);
-          if (!sawSIGINT) {
-            cb(null, result);
-          }
+          result.then(function(resolvedResult) {
+            if (!sawSIGINT) {
+              return cb(null, resolvedResult);
+            }
+          });
           return sawSIGINT = false;
         } else {
           return cb(null, result);