Fix bug in the requested type of an SSA instruction: avoid requesting a ContainerTypeMask.

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

Index: sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart
===================================================================
--- sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart	(revision 27940)
+++ sdk/lib/_internal/compiler/implementation/ssa/types_propagation.dart	(working copy)
@@ -397,7 +397,10 @@
 
   HType computeDesiredTypeForInput(HInstruction user, HInstruction input) {
     this.input = input;
-    HType desired = user.accept(this);
+    // We simplify the desired type to avoid requesting the type of an
+    // instantiation node, for example [ContainerTypeMask].
+    HType desired = user.accept(this).simplify(compiler);
+    assert(!desired.computeMask(compiler).isContainer);
     this.input = null;
     return desired;
   }