make tensor instanceof more robust

[tafsiri]

This adds a workaround for how babel compiles our classes, and should fix #3384.

The main issue is that babel will try and run instanceof on a tensor before our constructor body has executed. This works around this by checking for the presence of methods on Tensor.

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This change is 

[annxingyuan]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan and @tafsiri)

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tfjs-core/src/tensor.ts, line 446 at r1 (raw file):

Object.defineProperty(Tensor, Symbol.hasInstance, {
  value: (instance: Tensor) => {
    // Implementation note: we should use properties of the object will be

of the object that will be defined

[tafsiri]

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, and @nsthorat)

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tfjs-core/src/tensor.ts, line 446 at r1 (raw file):

Previously, annxingyuan (Ann Yuan) wrote…

of the object that will be defined

done