David Ye 15b7f25a65 Add property length to SortedSet (#42872) ...

The SortedSet [documentation](https://www.collectionsjs.com/sorted-set) indicates that it has a property `length`.

2020-03-27 13:10:04 -04:00

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collections-tests.ts

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index.d.ts

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sorted-set.d.ts

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tsconfig.json

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tslint.json

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