replace instanceof Promise with check of then function on the object

[pyu10055]

In Angular Promise objects are monkey patched for zone.js
The type check object instanceof Promise will return false.
This PR created a util method that check the type of then field of the object instead.

fix #3494

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This change is 

[lina128]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @lina128 and @pyu10055)

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tfjs-core/src/util.ts, line 800 at r1 (raw file):

/**
 * This method to check of the object can be interpreted as `Promise` instance.

This method asserts whether an object is a Promise instance.

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tfjs-core/src/util.ts, line 802 at r1 (raw file):

 * Since `instanceof Promise` only works for es6 `Promise`, some framework like
 * Angular has custom promise where `instanceof Promise` return false.

Should we move this part inside the function? Because it is implementation detail.

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tfjs-core/src/util.ts, line 806 at r1 (raw file):

 */
export function isPromise(object: any) {
  return object && object.then && typeof object.then === 'function';

Add the annotation here, something like:
// We chose to not use 'obj instanceOf Promise' for two reasons: 1. It only reliably works for es6 Promise, not other Promise implementations. 2. It doesn't work with framework that uses zone.js. zone.js monkey patch the async calls, so it is possible the obj (patched) is comparing to a pre-patched Promise.

[pyu10055]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @lina128)

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tfjs-core/src/util.ts, line 800 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

This method asserts whether an object is a Promise instance.

Done.

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tfjs-core/src/util.ts, line 802 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

 * Since `instanceof Promise` only works for es6 `Promise`, some framework like
 * Angular has custom promise where `instanceof Promise` return false.

Should we move this part inside the function? Because it is implementation detail.

Done.

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tfjs-core/src/util.ts, line 806 at r1 (raw file):

Previously, lina128 (Na Li) wrote…

Add the annotation here, something like:
// We chose to not use 'obj instanceOf Promise' for two reasons: 1. It only reliably works for es6 Promise, not other Promise implementations. 2. It doesn't work with framework that uses zone.js. zone.js monkey patch the async calls, so it is possible the obj (patched) is comparing to a pre-patched Promise.

Done.