Protect types_propagation receiver type strengthening against many users.

pkg/compiler/lib/src/ssa/nodes.dart

Index: pkg/compiler/lib/src/ssa/nodes.dart
diff --git a/pkg/compiler/lib/src/ssa/nodes.dart b/pkg/compiler/lib/src/ssa/nodes.dart
index 6ed6aefd82b331c399289af6204eec97906fcbe9..929cb7751ad05f1bbb47dc5720e1a3a9339d8eee 100644
--- a/pkg/compiler/lib/src/ssa/nodes.dart
+++ b/pkg/compiler/lib/src/ssa/nodes.dart
@@ -1262,8 +1262,9 @@ abstract class HInstruction implements Spannable {
     HBasicBlock otherBlock = other.block;
     for (int i = 0, length = usedBy.length; i < length; i++) {
       HInstruction current = usedBy[i];
-      if (otherBlock.dominates(current.block)) {
-        if (identical(current.block, otherBlock)) usersInCurrentBlock++;
+      HBasicBlock currentBlock = current.block;
+      if (otherBlock.dominates(currentBlock)) {
+        if (identical(currentBlock, otherBlock)) usersInCurrentBlock++;
         users.add(current);
       }
     }