Observations

https://accidentallyquadratic.tumblr.com/

In short, a concatenation of thousands of strings may be costly if done naively (O(n^2)); JavaScript people knew that, Java people knew that.
Java fix was implemented at least several years ago; somebody did something for JavaScript a while ago, as far as I know.

Now it is year 2019, and Neal Gafter fixes a C# compiler to do the stuff right - this time by using Ropes - good for him.

People are celebrating.

And I wonder... what's wrong with us... Was not it Neal that fixed Java's "abc"+"cde" about 12 years ago? Or was it Josh?