Update flow to 0.59 and fix some flow issues (#6957)

packages/babylon/src/parser/statement.js

@@ -1513,10 +1513,9 @@ export default class StatementParser extends ExpressionParser {
     }
   }
 
-  checkDeclaration(node: N.Pattern): void {
+  checkDeclaration(node: N.Pattern | N.ObjectProperty): void {
     if (node.type === "ObjectPattern") {
       for (const prop of node.properties) {
-        // $FlowFixMe (prop may be an AssignmentProperty, in which case this does nothing?)
         this.checkDeclaration(prop);
       }
     } else if (node.type === "ArrayPattern") {

packages/babylon/src/plugins/estree.js

@@ -74,10 +74,9 @@ export default (superClass: Class<Parser>): Class<Parser> =>
       node.expression = false;
     }
 
-    checkDeclaration(node: N.Pattern): void {
+    checkDeclaration(node: N.Pattern | N.ObjectProperty): void {
       if (isSimpleProperty(node)) {
-        // $FlowFixMe
-        this.checkDeclaration(node.value);
+        this.checkDeclaration(((node: any): N.EstreeProperty).value);
       } else {
         super.checkDeclaration(node);
       }