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* Use persistent hash map to store _inheritedWidgets Instead of using a HashMap and copying it down the tree which leads to quadratic time and space complexity use a persistent data structure which can amortize the cost by sharing parts of the structure. The data shows HAMT based PersistentHashMap to be 5-10x faster for building _inheritedWidgets and considerably more space effecient (e.g. bringing amount of memory allocated when constructing _inheritedWidgets in a tree with 150 InheritedWidget down to 70Kb from 970Kb). PersistentHashMap is slower than HashMap for access: 2-3x in relative terms, but in absolute terms we are only talking about ~0.2ns slow down per access and various app benchmarks we run have have not revealed any significant regressions.