Trust inferred type when propagating types in the SSA backend.

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

Index: pkg/compiler/lib/src/ssa/types_propagation.dart
diff --git a/pkg/compiler/lib/src/ssa/types_propagation.dart b/pkg/compiler/lib/src/ssa/types_propagation.dart
index 71abe6c3ed5bfa4df8dbb65ad9df38538f69b6fe..0aa5f0fe9bb3c53d3c058c43b40a5c2edf41c138 100644
--- a/pkg/compiler/lib/src/ssa/types_propagation.dart
+++ b/pkg/compiler/lib/src/ssa/types_propagation.dart
@@ -343,10 +343,10 @@ class SsaTypePropagator extends HBaseVisitor implements OptimizationPhase {
     }
 
     HInstruction receiver = instruction.getDartReceiver(compiler);
-    TypeMask receiverType = receiver.instructionType;
-    Selector selector =
-        new TypedSelector(receiverType, instruction.selector, classWorld);
-    instruction.selector = selector;
+    Selector selector = instruction.selector;
+    TypeMask receiverType = selector is TypedSelector
+        ? selector.mask.intersection(receiver.instructionType, classWorld)
+        : receiver.instructionType;
 
     // Try to specialize the receiver after this call.
     if (receiver.dominatedUsers(instruction).length != 1