The "heapify" algorithm has linear performance for creating a heap from an existing list.
It won't work directly for addAll if the heap isn't empty.
It can add all the new elements to the end of the heap, and then run the heapify algorithm on the entire heap. That may not be faster than just bubbling up the new elements, but that depends on the number of elements added.
I don't know if it's meaningful to adapt the algorithm to knowing that some elements are already heap-ordered. Probably too expensive, the complexity of heapify is very low.

We should have the heapify algorithm, could add a .of constructor to initialize with it, and maybe use it for addAll if it's expected to be faster than just saying each new element independently.

Adding M elements individually to a heap of size N has time O(M×log(M+N)), adding and heapifying has time O(M+N). The latter is better if log2(M+N) > (M+N)/M. A doubling or more (N<=M) will always satisfy that for non trivial sizes (N >= 2). Less may also be OK, but then we should probably consider the constant factor too. If we less than double, we can probably approximate N+M with just N, so it's log2(N) > N/M, or M > N/log2(N).

Or even more approximate and in Dart, m * n.bitCount > n. That seems efficient enough to use as a test for whether to bubble all the new elements up, or heapify the entire heap.

LGTM.