VM: Propagate non-nullness from instance calls.

runtime/vm/intermediate_language.cc

Index: runtime/vm/intermediate_language.cc
diff --git a/runtime/vm/intermediate_language.cc b/runtime/vm/intermediate_language.cc
index 3d7c22c59de7061a2b080e294ac72946d5c08895..054111505811a1f05621d64b4635ae89881dd4d3 100644
--- a/runtime/vm/intermediate_language.cc
+++ b/runtime/vm/intermediate_language.cc
@@ -1938,6 +1938,18 @@ Definition* Definition::Canonicalize(FlowGraph* flow_graph) {
 }
 
 
+Definition* RedefinitionInstr::Canonicalize(FlowGraph* flow_graph) {
+  if (!HasUses()) {
+    return NULL;
+  }
+  if ((constrained_type() != NULL) &&
+      Type()->IsEqualTo(value()->definition()->Type())) {
+    return value()->definition();
+  }
+  return this;
+}
+
+
 bool LoadFieldInstr::IsImmutableLengthLoad() const {
   switch (recognized_kind()) {
     case MethodRecognizer::kObjectArrayLength: