Always output comment newlines, fix edge case with function param output

src/comments.js

@@ -6,7 +6,7 @@ var isObject = types.builtInTypes.object;
 var pp = require("./pp");
 var fromString = pp.fromString;
 var concat = pp.concat;
-var line = pp.line;
+var hardline = pp.hardline;
 var util = require("./util");
 var comparePos = util.comparePos;
 var childNodesCacheKey = require("private").makeUniqueKey();
@@ -266,7 +266,7 @@ function printLeadingComment(commentPath, print) {
   if (comment.trailing) {
     // When we print trailing comments as leading comments, we don't
     // want to bring any trailing spaces along.
-    parts.push(line);
+    parts.push(hardline);
 
   }
 
@@ -288,7 +288,7 @@ function printLeadingComment(commentPath, print) {
 
   // }
   else {
-    parts.push(line);
+    parts.push(hardline);
   }
 
   return concat(parts);

src/printer.js

@@ -1740,17 +1740,19 @@ function printArgumentsList(path, options, print) {
     return "()";
   }
 
-  const shouldBreak = printed.slice(0, -1).some(hasHardLine);
   const lastArg = util.getLast(path.getValue().arguments);
   // This is just an optimization; I think we could return the
   // conditional group for all function calls, but it's more expensive
   // so only do it for specific forms.
   const groupLastArg = lastArg.type === "ObjectExpression" ||
         lastArg.type === "ArrayExpression" ||
         lastArg.type === "FunctionExpression" ||
-        lastArg.type === "ArrowFunctionExpression";
+        lastArg.type === "ArrowFunctionExpression" ||
+        lastArg.type === "CallExpression" ||
+        lastArg.type === "NewExpression";
 
   if (groupLastArg) {
+    const shouldBreak = printed.slice(0, -1).some(hasHardLine);
     return conditionalGroup(
       [
         concat([
@@ -1784,6 +1786,7 @@ function printArgumentsList(path, options, print) {
     );
   }
 
+  const shouldBreak = printed.some(hasHardLine);
   return group(
     concat([
       "(",