Add parenthesis around assignment for arrow function body (#422)

Fixes #412

src/fast-path.js

@@ -376,6 +376,12 @@ FPp.needsParens = function(assumeExpressionContext) {
         parent.object === node;
 
     case "AssignmentExpression":
+      if (parent.type === "ArrowFunctionExpression" &&
+          parent.body === node &&
+          node.left.type === "ObjectPattern") {
+        return true;
+      }
+
     case "ConditionalExpression":
       switch (parent.type) {
         case "TaggedTemplateExpression":

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

@@ -64,6 +64,8 @@ exports[`test arrows.js 1`] = `
     return images.find(image => image.width >= maxPixelWidth) ||
         images[images.length - 1];
 }
+
+a = () => ({} = this);
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 function selectBestEffortImageForWidth(
   maxWidth: number,
@@ -75,6 +77,8 @@ function selectBestEffortImageForWidth(
   return images.find(image => image.width >= maxPixelWidth) ||
     images[images.length - 1];
 }
+
+a = () => ({} = this);
 "
 `;
 

tests/arrows/arrows.js

@@ -8,3 +8,5 @@ function selectBestEffortImageForWidth(
     return images.find(image => image.width >= maxPixelWidth) ||
         images[images.length - 1];
 }
+
+a = () => ({} = this);