function newReadableStream(underlyingSource = {}, strategy = {}) {
  if (USE_NATIVE_IMPLEMENTATION === 'true' ||
      (USE_NATIVE_IMPLEMENTATION === 'detect' && goog.global.ReadableStream)) {
    return fullNativeImpl.newReadableStream(underlyingSource, strategy);
  } else {
    return fullImpl.newReadableStream(underlyingSource, strategy);
  }
}
Creates and returns a new ReadableStream.

The underlying source should only have a start() method, and no other
properties.
@param {!ReadableStreamUnderlyingSource<T>=} underlyingSource
@param {!ReadableStreamStrategy<T>=} strategy
@return {!ReadableStream<T>}
@suppress {strictMissingProperties}
@template T