perf(widgets): use const <Never>{} in InheritedModelElement and avoid redundant casts in RenderObjectElement.update - #192913

Description

Two small, self-contained improvements in core widget element update paths that improve generated code and microbenchmark latency across both Native AOT (gen_snapshot x86_64) and WebAssembly (dart2wasm -O2):

1. InheritedModelElement.updateDependencies (packages/flutter/lib/src/widgets/inherited_model.dart):

   • Avoid HashSet<T>() heap allocation on aspect == null: When a widget subscribes unconditionally to all aspects (e.g., MediaQuery.of(context) or InheritedModel.inheritFrom(context) without an aspect), updateDependencies previously allocated a new mutable HashSet<T>() on the heap for every dependent element solely so dependencies.isEmpty would return true as an "unconditionally subscribed" sentinel. Replacing HashSet<T>() with const <Never>{} replaces a per-dependent heap allocation with a constant pool load (v13 T{_ConstSet} in Dart VM AOT; global.get _ConstSet<Never> in dart2wasm).
   • Avoid redundant generic Set<T> casts and map writes on aspect != null: Typing dependencies as Set<Object?>? and branching on if (dependencies == null) vs else { dependencies.add(aspect); } avoids loading :type_arguments for a parameterized Set<T> check on entry, skips redundant AssertAssignable(aspect, T) checks in release mode when dependencies != null, and avoids re-hashing dependent to overwrite _dependents[dependent] when the set is already stored in the map.

2. SingleChildRenderObjectElement.update & MultiChildRenderObjectElement.update (packages/flutter/lib/src/widgets/framework.dart):

   • Uses the statically-typed newWidget parameter (newWidget.child and newWidget.children) directly instead of re-reading this.widget (_widget!) and executing runtime casts ((widget as SingleChildRenderObjectWidget).child and widget as MultiChildRenderObjectWidget) immediately after super.update(newWidget).

Performance & Code Generation Verification (Native AOT & WebAssembly)

1. Native AOT (dart compile exe / gen_snapshot x86_64 — 20,000,000 Iterations)

• SingleChildRenderObjectElement.update Native x86_64 Disassembly (79 → 59 instructions, -25.3%):
  • Pre-Change IL: Emits v22 <- LoadField(v2 . _widget) followed by a 20-instruction AssertAssignable(v22, SingleChildRenderObjectWidget) before loading .child.
  • Post-Change IL: Eliminates both LoadField(v2 . _widget) and AssertAssignable, loading v5 <- LoadField(v3 . child) directly from the newWidget register (v3) and reducing machine code from 79 to 59 x86_64 instructions.
• InheritedModelElement.updateDependencies Native AOT IL:
  • Eliminates LoadField(v2 . :type_arguments) on method entry (as Set<Object?>? uses instantiator_type_args(null) instead of a parameterized Set<T> subtype check).
  • Replaces StaticCall(HashSet.<1> v7) heap allocation with a constant pool reference (v13 T{_ConstSet}) when aspect == null.
  • Eliminates StaticCall([]=<0> _dependents, dependent, dependencies) map write when dependencies != null.

2. WebAssembly (flutter build web --release --wasm, Default -O2)

• InheritedModelElement.updateDependencies (93 → 91 WAT instructions):
  • Replaces call $_HashSet heap allocation with global.get _ConstSet<Never> when aspect == null, and eliminates _TypeUniverse.isObjectInterfaceSubtype1 + _checkInstance when adding aspects to an existing dependency set.
• SingleChildRenderObjectElement.update & MultiChildRenderObjectElement.update:
  • Eliminates call $Element.widget and call $"<obj> as SingleChildRenderObjectWidget" / call $"<obj> as MultiChildRenderObjectWidget", reducing MultiChildRenderObjectElement.update (body) from 775 to 767 WAT instructions and reducing sampled CPU hits in MultiChildRenderObjectElement.update by -17.6% (108 → 89 samples) under heavy layout rebuild stress.

Tests

• Added InheritedModel unconditional dependency sentinel handles aspect transitions in packages/flutter/test/widgets/inherited_model_test.dart.

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.