Call special VM interpolate for single element interpolation.

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/class_finalizer.h"
@@ skipping 2368 matching lines @@
       Do(store);
     }
     ReturnDefinition(ExitTempLocalScope(tmp_var));
   }
 }
 
 
 void EffectGraphVisitor::VisitStringInterpolateNode(
     StringInterpolateNode* node) {
   ValueGraphVisitor for_argument(owner());
-  node->value()->Visit(&for_argument);
+  ArrayNode* arguments = node->value();
+  bool is_singleton = false;
+  if (arguments->length() == 1) {
+    arguments->ElementAt(0)->Visit(&for_argument);
+    is_singleton = true;
+  } else {
+    arguments->Visit(&for_argument);
+  }
   Append(for_argument);
   StringInterpolateInstr* instr =
-      new(I) StringInterpolateInstr(for_argument.value(), node->token_pos());
+      new(I) StringInterpolateInstr(for_argument.value(), node->token_pos(),
+                                    is_singleton);
   ReturnDefinition(instr);
 }
 
 
 void EffectGraphVisitor::VisitClosureNode(ClosureNode* node) {
   const Function& function = node->function();
 
   if (function.IsImplicitStaticClosureFunction()) {
     const Instance& closure =
         Instance::ZoneHandle(I, function.ImplicitStaticClosure());
@@ skipping 1820 matching lines @@
   Report::MessageF(Report::kBailout,
                    Script::Handle(function.script()),
                    function.token_pos(),
                    "FlowGraphBuilder Bailout: %s %s",
                    String::Handle(function.name()).ToCString(),
                    reason);
   UNREACHABLE();
 }
 
 }  // namespace dart