[vm] Remove double-conversion's dependency on the STL's locale.

out/ReleaseX64/exe.stripped/dartaotruntime -187k (-3.14%)

TEST=ci
Change-Id: I71652fe4c090132fcf938f8865f9c50594bf8cfd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/483622
Reviewed-by: Alexander Aprelev <aam@google.com>
Commit-Queue: Ryan Macnak <rmacnak@google.com>

third_party/double-conversion/README.dart

@@ -9,3 +9,4 @@ for v8.
 
 Local Modifications:
 Removed the test directory and the build files.
+Removed dependency on <locale>.

third_party/double-conversion/src/string-to-double.cc

@@ -26,7 +26,6 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #include <climits>
-#include <locale>
 #include <cmath>
 
 #include "string-to-double.h"
@@ -52,9 +51,8 @@ namespace double_conversion {
 namespace {
 
 inline char ToLower(char ch) {
-  static const std::ctype<char>& cType =
-      std::use_facet<std::ctype<char> >(std::locale::classic());
-  return cType.tolower(ch);
+  if (ch >= 'A' && ch <= 'Z') return ch - 'A' + 'a';
+  return ch;
 }
 
 inline char Pass(char ch) {