Prevent hoisting of certain check nodes, including [HTypeKnown].

sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart

Index: sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart b/sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart
index 8bde7edb2d8e69673e782f06200f5bc0090c38a0..75fe4b67f3c729097362b08d36482fd4086d5740 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart
@@ -343,7 +343,8 @@ class SsaTypePropagator extends HBaseVisitor implements OptimizationPhase {
         // Insert a refinement node after the call and update all
         // users dominated by the call to use that node instead of
         // [receiver].
-        HTypeKnown converted = new HTypeKnown(newType, receiver);
+        HTypeKnown converted =
+            new HTypeKnown.witnessed(newType, receiver, instruction);
         instruction.block.addBefore(instruction.next, converted);
         receiver.replaceAllUsersDominatedBy(converted.next, converted);
         addDependentInstructionsToWorkList(converted);