Fix use strict as expression statement (#602)

* Add parentheses if node is StringLiteral surrounded by ExpressionStatement :tools:.

* Add new test.

* Update tests to run against Babylon, plus no regression case.

* Split actual test and no regression test in two files.

* Disable Flow till facebook/flow#3234 is merged.

src/fast-path.js

@@ -497,6 +497,11 @@ FPp.needsParens = function(assumeExpressionContext) {
         return name === "object" && parent.object === node;
       }
 
+    case "StringLiteral":
+      if (parent.type === "ExpressionStatement") {
+        return true;
+      }
+
     default:
       if (
         parent.type === "NewExpression" &&

tests/expression_statement/__snapshots__/jsfmt.spec.js.snap

@@ -0,0 +1,18 @@
+exports[`test no_regression.js 1`] = `
+"// Ensure no regression.
+\"use strict\";
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Ensure no regression.
+\"use strict\";
+
+"
+`;
+
+exports[`test use_strict.js 1`] = `
+"// Parentheses around expression statement should be preserved in this case.
+(\"use strict\");
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+// Parentheses around expression statement should be preserved in this case.
+(\"use strict\");
+"
+`;

tests/expression_statement/jsfmt.spec.js

@@ -0,0 +1,2 @@
+// TODO: Re-enable Flow when the following fix is merged facebook/flow#3234.
+run_spec(__dirname, {parser: 'babylon'});

tests/expression_statement/no_regression.js

@@ -0,0 +1,2 @@
+// Ensure no regression.
+"use strict";

tests/expression_statement/use_strict.js

@@ -0,0 +1,2 @@
+// Parentheses around expression statement should be preserved in this case.
+("use strict");