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kotlinlang

Hello everyone. I ran into a small issue using `measureTimeMillis`. I need to run some code, whose return value needs to be captured (roughly like the following snippet)
```
var result: T? = null
val time = measureTimeMillis { result = block() }
log.verbose("timing: $time")
return result!!
```
Declaring `result` as a nullable `var` (which implies the not-null-assertion in the end) seems unnecessary as `measureTimeMillis` will be inlined yielding roughly:
```
val result: T
...
result = block()
...
```
which is perfectly valid.

One possible solution is to create something like `measureTimeMillisWithResult` which returns the result and the timing. From my point of view a cleaner approach is to reuse the already existing `measureTimeMillis` for this, but augment it with
```
contract {
    callsInPlace(block, InvocationKind.EXACTLY_ONCE)
}
```
which should give the compiler enough hints to deduce that there's just one assignment.

While looking at it, `measureTimeMillis` could also be rewritten to delegate to `measureNanoTime` with some conversion, which could maybe fix <https://youtrack.jetbrains.com/issue/KT-24431>.

Is it worth creating a PR for this?