diff --git a/runtime/lib/string.cc b/runtime/lib/string.cc
index 486ff33a687..50636fc88f0 100644
--- a/runtime/lib/string.cc
+++ b/runtime/lib/string.cc
@@ -117,6 +117,27 @@ DEFINE_NATIVE_ENTRY(String_concat, 2) {
 }
 
 
+// TODO(hausner): Remove obsolete String_plus.
+DECLARE_FLAG(bool, allow_string_plus);
+DEFINE_NATIVE_ENTRY(String_plus, 2) {
+  const String& receiver = String::CheckedHandle(arguments->At(0));
+  GET_NATIVE_ARGUMENT(String, b, arguments->At(1));
+  if (!FLAG_allow_string_plus) {
+    // Throw a noSuchMethod exception if operator + is not supported.
+    const String& func_name = String::Handle(String::New("+"));
+    const Array& func_args = Array::Handle(Array::New(1));
+    func_args.SetAt(0, b);
+    GrowableArray<const Object*> dart_arguments(3);
+    dart_arguments.Add(&receiver);
+    dart_arguments.Add(&func_name);
+    dart_arguments.Add(&func_args);
+    Exceptions::ThrowByType(Exceptions::kNoSuchMethod, dart_arguments);
+  }
+  const String& result = String::Handle(String::Concat(receiver, b));
+  arguments->SetReturn(result);
+}
+
+
 DEFINE_NATIVE_ENTRY(String_toLowerCase, 1) {
   const String& receiver = String::CheckedHandle(arguments->At(0));
   ASSERT(!receiver.IsNull());
diff --git a/runtime/lib/string.dart b/runtime/lib/string.dart
index 7bcbda14023..c2b5579954c 100644
--- a/runtime/lib/string.dart
+++ b/runtime/lib/string.dart
@@ -232,12 +232,18 @@ class StringBase {
     return buffer.add(this.substring(startIndex)).toString();
   }
 
+  /**
+   * TODO(hausner): remove support for operator + and native method plus().
+   */
+  String plus(String other) native "String_plus";
+
   /**
    * Convert argument obj to string and concat it with this string.
    * Returns concatenated string.
+   * TODO(hausner): remove support for +.
    */
   String operator +(Object obj) {
-    return this.concat(obj.toString());
+    return this.plus(obj.toString());
   }
 
   /**
diff --git a/runtime/vm/bootstrap_natives.h b/runtime/vm/bootstrap_natives.h
index 040a2272be6..631c5d432e2 100644
--- a/runtime/vm/bootstrap_natives.h
+++ b/runtime/vm/bootstrap_natives.h
@@ -76,6 +76,7 @@ namespace dart {
   V(String_charAt, 2)                                                          \
   V(String_charCodeAt, 2)                                                      \
   V(String_concat, 2)                                                          \
+  V(String_plus, 2)                                                            \
   V(String_toLowerCase, 1)                                                     \
   V(String_toUpperCase, 1)                                                     \
   V(Strings_concatAll, 1)                                                      \
diff --git a/runtime/vm/parser.cc b/runtime/vm/parser.cc
index d69ba1e3446..cb35ec3e632 100644
--- a/runtime/vm/parser.cc
+++ b/runtime/vm/parser.cc
@@ -26,6 +26,7 @@ DEFINE_FLAG(bool, enable_type_checks, false, "Enable type checks.");
 DEFINE_FLAG(bool, trace_parser, false, "Trace parser operations.");
 DEFINE_FLAG(bool, warning_as_error, false, "Treat warnings as errors.");
 DEFINE_FLAG(bool, silent_warnings, false, "Silence warnings.");
+DEFINE_FLAG(bool, allow_string_plus, true, "Allow + operator on strings.");
 
 static void CheckedModeHandler(bool value) {
   FLAG_enable_asserts = value;
@@ -5702,8 +5703,12 @@ AstNode* Parser::ParseBinaryExpr(int min_preced) {
           if (left_operand->IsLiteralNode()) {
             LiteralNode* lit = left_operand->AsLiteralNode();
             if (lit->literal().IsString()) {
-              str_concat = new StringConcatNode(lit->token_index());
-              str_concat->AddExpr(lit);
+              if (FLAG_allow_string_plus) {
+                str_concat = new StringConcatNode(lit->token_index());
+                str_concat->AddExpr(lit);
+              } else {
+                ErrorMsg(op_pos, "operator + on strings no longer allowed");
+              }
             }
           } else if (left_operand->IsStringConcatNode()) {
             str_concat = left_operand->AsStringConcatNode();