Let flutter build aot select a target platform (#4154)

Currently only android-arm and ios are supported target platforms.
diff --git a/packages/flutter_tools/lib/src/build_info.dart b/packages/flutter_tools/lib/src/build_info.dart
index 78fcbf4..efa53c1 100644
--- a/packages/flutter_tools/lib/src/build_info.dart
+++ b/packages/flutter_tools/lib/src/build_info.dart
@@ -69,6 +69,24 @@
   assert(false);
 }
 
+TargetPlatform getTargetPlatformForName(String platform) {
+  switch (platform) {
+    case 'android-arm':
+      return TargetPlatform.android_arm;
+    case 'android-x64':
+      return TargetPlatform.android_x64;
+    case 'android-x86':
+      return TargetPlatform.android_x86;
+    case 'ios':
+      return TargetPlatform.ios;
+    case 'darwin-x64':
+      return TargetPlatform.darwin_x64;
+    case 'linux-x64':
+      return TargetPlatform.linux_x64;
+  }
+  return null;
+}
+
 HostPlatform getCurrentHostPlatform() {
   if (Platform.isMacOS)
     return HostPlatform.darwin_x64;