Add tf.util.fetch.

[annxingyuan]

This PR fixes tensorflow/tfjs#1439

Changes

• Added tf.util.fetch
• Add node-fetch as dependency
• Remove code that looks for global fetch in BrowserHTTPRequest

---

This change is 

[nsthorat]

Reviewed 5 of 9 files at r1.
Reviewable status: 0 of 1 approvals obtained (waiting on @annxingyuan, @dsmilkov, and @nsthorat)

---

package.json, line 56 at r1 (raw file):

    "test": "karma start",
    "run-browserstack": "karma start --browserstack",
    "test-benchmark": "cd integration_tests/benchmarks && yarn benchmark-travis && cd ../../",

do you mean to be deleting this here?

---

src/util.ts, line 671 at r1 (raw file):

/**
 * Returns a platform-specific implementation of `window.fetch`.

can you link to https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API

---

src/util.ts, line 682 at r1 (raw file):

 *  .then(response => {}) // handle response
 * ```
 */

@doc annotate this inside util? https://js.tensorflow.org/api/latest/#Util

---

src/util.ts, line 691 at r1 (raw file):

    if (ENV.get('IS_NODE')) {
      // tslint:disable-next-line:no-require-imports
      fetchFunc = require('node-fetch');

if we call tf.util.fetch multiple times, do we have to keep re-requiring this? can we cache this?

---

src/util.ts, line 693 at r1 (raw file):

      fetchFunc = require('node-fetch');
    } else {
      throw new Error(`Unable to find fetch polyfill.`)

semicolon

[annxingyuan]

Reviewable status: 0 of 1 approvals obtained (waiting on @annxingyuan and @dsmilkov)

---

src/util.ts, line 671 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

can you link to https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API

Done

---

src/util.ts, line 682 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

@doc annotate this inside util? https://js.tensorflow.org/api/latest/#Util

Done

---

src/util.ts, line 691 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

if we call tf.util.fetch multiple times, do we have to keep re-requiring this? can we cache this?

Done

---

src/util.ts, line 693 at r1 (raw file):

Previously, nsthorat (Nikhil Thorat) wrote…

semicolon

Done

[annxingyuan]

Closed in favor of #1663