Accept constant string concatenation and constant string length as constant

runtime/vm/ast.cc

Index: runtime/vm/ast.cc
===================================================================
--- runtime/vm/ast.cc	(revision 43830)
+++ runtime/vm/ast.cc	(working copy)
@@ -324,7 +324,7 @@
   if (left_val == NULL) {
     return NULL;
   }
-  if (!left_val->IsNumber() && !left_val->IsBool()) {
+  if (!left_val->IsNumber() && !left_val->IsBool() && !left_val->IsString()) {
     return NULL;
   }
   const Instance* right_val = this->right()->EvalConstExpr();
@@ -333,6 +333,10 @@
   }
   switch (kind_) {
     case Token::kADD:
+      if (left_val->IsString()) {
+        return right_val->IsString() ? left_val : NULL;
+      }
+      // Fall-through intentional.
     case Token::kSUB:
     case Token::kMUL:
     case Token::kDIV:
@@ -531,6 +535,23 @@
 }
 
 
+bool InstanceGetterNode::IsPotentiallyConst() const {
+  return field_name().Equals(Symbols::Length()) &&
+    receiver()->IsPotentiallyConst();
+}
+
+
+const Instance* InstanceGetterNode::EvalConstExpr() const {
+  if (field_name().Equals(Symbols::Length())) {
+    const Instance* receiver_val = receiver()->EvalConstExpr();
+    if ((receiver_val != NULL) && receiver_val->IsString()) {
+      return &Instance::ZoneHandle(Smi::New(1));
+    }
+  }
+  return NULL;
+}
+
+
 AstNode* LoadIndexedNode::MakeAssignmentNode(AstNode* rhs) {
   return new StoreIndexedNode(token_pos(), array(), index_expr(),
                               rhs, super_class());