Address STLPort-specific workarounds (#493)

* Use std::to_string (STLport is no longer supported).

* Remove more STLPort workarounds.

* Still can't use std::to_string

Author: ehsannas

firestore/src/common/field_value.cc

@@ -27,7 +27,7 @@ using Type = FieldValue::Type;
 
 template <typename T>
 std::string ValueToString(const T& value) {
-  // TODO(stlport): use `std::to_string` once possible.
+  // TODO(b/192885139): use `std::to_string` once possible.
   std::ostringstream stream;
   stream << value;
   return stream.str();

firestore/src/common/firestore.cc

@@ -132,8 +132,7 @@ Firestore* Firestore::AddFirestoreToCache(Firestore* firestore,
     return nullptr;
   }
 
-  // Once we remove STLPort support, change this back to `emplace`.
-  FirestoreCache()->insert(std::make_pair(firestore->app(), firestore));
+  FirestoreCache()->emplace(firestore->app(), firestore);
   return firestore;
 }
 

firestore/src/jni/env.cc

@@ -174,7 +174,7 @@ std::string Env::ToStringUtf(const String& string) {
   size_t len = GetArrayLength(bytes);
 
   // Copy directly into the std::string buffer. This is guaranteed to work as
-  // of C++11, and also happens to work with STLPort.
+  // of C++11.
   std::string result;
   result.resize(len);
 

firestore/src/jni/env.h

@@ -23,7 +23,7 @@ namespace firebase {
 namespace firestore {
 namespace jni {
 
-// Since we're targeting STLPort, `std::invoke` is not available.
+// TODO: Consider using `std::invoke` when we move to C++17.
 #define INVOKE(env, method, ...) ((env)->*(method))(__VA_ARGS__);
 
 /**