[flow-api-translator] Fix translated return type of async functions

[robhogan]

Summary:
Currently, flowToFlowDef (and therefore flowToTSDef) assumes that a function with an unspecified return type returns void.

This isn't true of async functions, which always return a Promise.

eg:

export async function foo() {}

Translates to:

Before

declare export function foo(): void;,

After

declare export function foo(): Promise<void>;,

This fix also covers class methods, fields, object expressions and arrow functions.

Test plan:

yarn test flowToFlowDef
yarn run v1.22.22
$ NODE_OPTIONS="--experimental-vm-modules" jest flowToFlowDef
(node:54215) [MODULE_TYPELESS_PACKAGE_JSON] Warning: Module type of file:///Users/robhogan/w/flow/packages/flow-parser/dist/estree/TransformComponentSyntax.js is not specified and it doesn't parse as CommonJS.
Reparsing as ES module because module syntax was detected. This incurs a performance overhead.
To eliminate this warning, add "type": "module" to /Users/robhogan/w/flow/packages/flow-parser/package.json.
(Use `node --trace-warnings ...` to show where the warning was created)
(node:54215) ExperimentalWarning: VM Modules is an experimental feature and might change at any time
 PASS  flow-api-translator/__tests__/flowToFlowDef-test.js
  flowToFlowDef
    Comments
      ✓ maintain docblock (31 ms)
      ✓ maintain toplevel statement comments (3 ms)
    dependency walking
      ✓ strip unused function (2 ms)
      ✓ keep used TypeAlias (2 ms)
      ✓ keep chain of used TypeAliases (1 ms)
      ✓ keep used TypeAlias with many references (1 ms)
      ✓ strip unused but shadowed deps (2 ms)
      ✓ strip unused but shadowed deps (complex) (5 ms)
    optimization pass
      ✓ strip unused import defs (2 ms)
    ExportNamedDeclaration
      ✓ type specifier (1 ms)
      ✓ type specifier with source (1 ms)
      ✓ value specifier with source
      ✓ all with source (1 ms)
    ExportDefaultDeclaration
      ✓ export default function (1 ms)
      ✓ export default class (1 ms)
      ✓ export default expression (1 ms)
      ✓ export default var
      ✓ export default object expression (2 ms)
      ✓ export default member expression (1 ms)
      ✓ export default object with member expression (1 ms)
    ExportAllDeclaration
      ✓ export basic (1 ms)
    module.exports
      ✓ export basic
    exports.*
      ✓ export basic (1 ms)
    FunctionDeclaration
      ✓ basic (1 ms)
      ✓ sync without return type
      ✓ async without return type
      ✓ with type params (1 ms)
      ✓ with params
      ✓ with rest params (1 ms)
      ✓ with destructured rest params (1 ms)
      ✓ with default params
      ✓ without identifier (1 ms)
      ✓ without identifier with default param (1 ms)
      ✓ with predicates (1 ms)
    TypeAlias
      ✓ basic (1 ms)
      ✓ with type params
    OpaqueType
      ✓ basic (1 ms)
      ✓ basic local (1 ms)
      ✓ with type params
      ✓ with super type (1 ms)
      ✓ with super type and type params
    ImportDeclaration
      ✓ basic (1 ms)
      ✓ type specifiers (1 ms)
    ClassDeclaration
      ✓ property (5 ms)
      ✓ method (5 ms)
      ✓ strips munged underscore members (1 ms)
      ✓ keeps double-underscore members
      ✓ keeps single-char underscore member (1 ms)
      ✓ strips static munged members
      ✓ strips munged members and their unused imports (1 ms)
      ✓ keeps munged members when mungeUnderscores is false (1 ms)
    InterfaceDeclaration
      ✓ property
      ✓ method (1 ms)
      ✓ local
    VariableDeclaration
      ✓ default require of class used as type (1 ms)
      ✓ default require of function used with typeof (1 ms)
      ✓ default require transitive dep (2 ms)
      ✓ default require member access (1 ms)
      ✓ destructured require all specifiers used
      ✓ destructured require strips unused specifiers
      ✓ destructured require with rename (1 ms)
      ✓ unused default require is stripped
      ✓ unused destructured require is stripped (1 ms)
      ✓ mixed default and destructured requires (1 ms)
      ✓ basic type parameter
      ✓ basic typecast (1 ms)
      ✓ prefer type parameter
      ✓ with dependency (1 ms)
      ✓ with imported value
      ✓ with object type (1 ms)
    EnumDeclaration
      ✓ basic (1 ms)
      ✓ local
    DeclareClass
      ✓ basic
      ✓ complex (1 ms)
      ✓ extends member expression (1 ms)
      ✓ extends type cast expression
      ✓ extends as cast expression (1 ms)
      ✓ extends type cast typeof expression (1 ms)
    Expression
      Identifier
        ✓ basic (1 ms)
      ObjectExpression
        ✓ empty
        ✓ methods (5 ms)
        ✓ properties (1 ms)
        ✓ spread (1 ms)
      Literals
        ✓ number (1 ms)
        ✓ string
        ✓ boolean (1 ms)
        ✓ regex (1 ms)
        ✓ null
      TypeCastExpression
        ✓ basic (1 ms)
      AsExpression
        ✓ basic
      FunctionExpression
        ✓ basic (2 ms)
      ArrowFunctionExpression
        ✓ basic (2 ms)
    ComponentDeclaration
      ✓ export (1 ms)
      ✓ export default
      ✓ params (1 ms)
      ✓ params with identifier-safe string literal names use individual params (1 ms)
      ✓ params with non-identifier string literal names use props object
      ✓ params with comments (1 ms)
      ✓ default params
      ✓ rest param (1 ms)
      ✓ params with rest param
      ✓ non-identifier string literal params with rest param (1 ms)
      ✓ non-identifier string literal params with typed rest param (1 ms)
      ✓ destructured rest param
      ✓ renders type
    HookDeclaration
      ✓ basic (1 ms)
      ✓ with params
      ✓ with type params (1 ms)
      ✓ without return type

Test Suites: 1 passed, 1 total
Tests:       109 passed, 109 total
Snapshots:   2 passed, 2 total
Time:        0.333 s, estimated 1 s
Ran all test suites matching flowToFlowDef.

[meta-codesync]

This pull request has been imported. If you are a Meta employee, you can view this in D117179919. (Because this pull request was imported automatically, there will not be any future comments.)

[meta-codesync]

This pull request has been merged in c4bbbb8.