Revert "Fix: check if param is assignable when parsing arrow return type annotation" (#12173)

This reverts commit 136e6301cb.

packages/babel-parser/src/parser/expression.js

@@ -1453,7 +1453,7 @@ export default class ExpressionParser extends LValParser {
     if (
       canBeArrow &&
       this.shouldParseArrow() &&
-      (arrowNode = this.parseArrow(arrowNode, exprList))
+      (arrowNode = this.parseArrow(arrowNode))
     ) {
       if (!this.isAwaitAllowed() && !this.state.maybeInAsyncArrowHead) {
         this.state.awaitPos = oldAwaitPos;
@@ -1509,10 +1509,7 @@ export default class ExpressionParser extends LValParser {
     return !this.canInsertSemicolon();
   }
 
-  parseArrow(
-    node: N.ArrowFunctionExpression,
-    exprList: N.Node[], // eslint-disable-line no-unused-vars
-  ): ?N.ArrowFunctionExpression {
+  parseArrow(node: N.ArrowFunctionExpression): ?N.ArrowFunctionExpression {
     if (this.eat(tt.arrow)) {
       return node;
     }

packages/babel-parser/src/parser/lval.js

@@ -51,59 +51,10 @@ export default class LValParser extends NodeUtils {
   +parseDecorator: () => Decorator;
   */
 
-  /**
-   * Check if a node can be converted to a binding identifier or binding pattern.
-   * https://tc39.es/ecma262/#prod-BindingIdentifier
-   * https://tc39.es/ecma262/#prod-BindingPattern
-   * Note that although a mebmer expression can serve as a LHS in the init of for loop,
-   * i.e. `for (a.b of []);`, it is not a binding pattern
-   *
-   * @param {Node} node
-   * @returns {boolean}
-   * @memberof LValParser
-   */
-  isAssignable(node: Node): boolean {
-    switch (node.type) {
-      case "Identifier":
-      case "ObjectPattern":
-      case "ArrayPattern":
-      case "AssignmentPattern":
-        return true;
-
-      case "ObjectExpression": {
-        const last = node.properties.length - 1;
-        return node.properties.every((prop, i) => {
-          return (
-            prop.type !== "ObjectMethod" &&
-            (i === last || prop.type === "SpreadElement") &&
-            this.isAssignable(prop)
-          );
-        });
-      }
-
-      case "ObjectProperty":
-        return this.isAssignable(node.value);
-
-      case "SpreadElement":
-        return this.isAssignable(node.argument);
-
-      case "ArrayExpression":
-        return node.elements.every(element => this.isAssignable(element));
-
-      case "AssignmentExpression":
-        return node.operator === "=";
-
-      case "ParenthesizedExpression":
-        return this.isAssignable(node.expression);
-
-      default:
-        return false;
-    }
-  }
-
   // Convert existing expression atom to assignable pattern
   // if possible.
-  // When this one is updated, please check if `isAssignable` also needs to be updated.
+  // NOTE: There is a corresponding "isAssignable" method in flow.js.
+  // When this one is updated, please check if also that one needs to be updated.
 
   toAssignable(node: Node): Node {
     let parenthesized = undefined;