[promise] Test IsPromise() early in Promise.prototype.then()

Addresses TODO by Dan --- simply by moving the check and exception
earlier in the function, before calling NewPromiseCapability() or
loading the constructor.

BUG=v8:4633
LOG=N
R=adamk@chromium.org, littledan@chromium.org, cbruni@chromium.org

Fixes 'test262/built-ins/Promise/prototype/then/context-check-on-entry'

Review URL: https://codereview.chromium.org/1561193002

Cr-Commit-Position: refs/heads/master@{#33137}

src/js/promise.js

@@ -274,16 +274,16 @@ function PromiseRejected(r) {
 // Multi-unwrapped chaining with thenable coercion.
 
 function PromiseThen(onResolve, onReject) {
+  var status = GET_PRIVATE(this, promiseStatusSymbol);
+  if (IS_UNDEFINED(status)) {
+    throw MakeTypeError(kNotAPromise, this);
+  }
+
   var constructor = this.constructor;
   onResolve = IS_CALLABLE(onResolve) ? onResolve : PromiseIdResolveHandler;
   onReject = IS_CALLABLE(onReject) ? onReject : PromiseIdRejectHandler;
   var deferred = NewPromiseCapability(constructor);
-  switch (GET_PRIVATE(this, promiseStatusSymbol)) {
-    case UNDEFINED:
-      // TODO(littledan): The type check should be called before
-      // constructing NewPromiseCapability; this is observable when
-      // erroneously copying this method to other classes.
-      throw MakeTypeError(kNotAPromise, this);
+  switch (status) {
     case 0:  // Pending
       GET_PRIVATE(this, promiseOnResolveSymbol).push(onResolve, deferred);
       GET_PRIVATE(this, promiseOnRejectSymbol).push(onReject, deferred);

test/test262/test262.status

@@ -415,7 +415,6 @@
 
   # https://bugs.chromium.org/p/v8/issues/detail?id=4633
   'built-ins/Promise/prototype/then/deferred-is-resolved-value': [SKIP],
-  'built-ins/Promise/prototype/then/context-check-on-entry': [FAIL],
   'built-ins/Promise/exception-after-resolve-in-executor': [FAIL],
   'built-ins/Promise/reject-function-name': [FAIL],
   'built-ins/Promise/reject-function-nonconstructor': [FAIL],