Improve walking of object patterns

This isn't great, as it's hard-coding node names of child nodes,
but the way the walker handles different contexts (expression,
statement, pattern) doesn't allow for a proper solution here.

Issue #688

src/walk/index.js

@@ -284,8 +284,12 @@ base.ArrayPattern = (node, st, c) => {
 }
 base.ObjectPattern = (node, st, c) => {
   for (let prop of node.properties) {
-    if (prop.computed) c(prop.key, st, "Expression")
-    c(prop.value, st, "Pattern")
+    if (prop.type === "Property") {
+      if (prop.computed) c(prop.key, st, "Expression")
+      c(prop.value, st, "Pattern")
+    } else if (prop.type === "RestElement") {
+      c(prop.argument, st, "Pattern")
+    }
   }
 }