Use VS 2017 and 2019 on CI, fix cast issue in dart_idl (#5740)

* vs2019

* another attempt

* syntax

* fewer

* fix dart

appveyor.yml

@@ -2,8 +2,6 @@ branches:
   only:
     - master
 
-os: Visual Studio 2015
-
 environment:
 
   global:
@@ -14,15 +12,26 @@ environment:
     CPP_TEST_OPTIONS: ""
 
   matrix:
-    - CMAKE_VS_VERSION: "10 2010"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      CMAKE_VS_VERSION: "10 2010"
       CMAKE_OPTIONS: "-DFLATBUFFERS_BUILD_LEGACY=1"
       CPP_TEST_OPTIONS: "--std-cpp c++0x"
       MONSTER_EXTRA: "skip"
 
-    - CMAKE_VS_VERSION: "12 2013"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      CMAKE_VS_VERSION: "12 2013"
       MONSTER_EXTRA: "skip"
 
-    - CMAKE_VS_VERSION: "14 2015"
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
+      CMAKE_VS_VERSION: "14 2015"
+      MONSTER_EXTRA: ""
+
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2017
+      CMAKE_VS_VERSION: "15 2017"
+      MONSTER_EXTRA: ""
+
+    - APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2019
+      CMAKE_VS_VERSION: "16 2019"
       MONSTER_EXTRA: ""
 
 platform:
@@ -37,7 +46,9 @@ before_build:
   - set MONSTER_EXTRA=%MONSTER_EXTRA%
   - cmake . -G"Visual Studio %CMAKE_VS_VERSION%" -DFLATBUFFERS_CODE_SANITIZE=1 %CMAKE_OPTIONS%
   # This cuts down on a lot of noise generated by xamarin warnings.
-  - del "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
+  - if exist "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets" del "C:\Program Files (x86)\MSBuild\14.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
+  - if exist "C:\Program Files (x86)\MSBuild\15.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets" del "C:\Program Files (x86)\MSBuild\15.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
+  - if exist "C:\Program Files (x86)\MSBuild\16.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets" del "C:\Program Files (x86)\MSBuild\16.0\Microsoft.Common.targets\ImportAfter\Xamarin.Common.targets"
 
 build:
   project: ALL_BUILD.vcxproj

src/idl_gen_dart.cpp

@@ -454,7 +454,7 @@ class DartGenerator : public BaseGenerator {
            it != struct_def.fields.vec.end(); ++it) {
         auto &field = **it;
         if (field.deprecated) continue;
-        auto offset = it - struct_def.fields.vec.begin();
+        auto offset = static_cast<int>(it - struct_def.fields.vec.begin());
         non_deprecated_fields.push_back(std::make_pair(offset, &field));
     }