Change the zone allocation api.

runtime/vm/double_conversion.cc

Index: runtime/vm/double_conversion.cc
===================================================================
--- runtime/vm/double_conversion.cc	(revision 10253)
+++ runtime/vm/double_conversion.cc	(working copy)
@@ -86,8 +86,7 @@
       kDoubleToStringCommonExponentChar,
       0, 0, 0, 0);  // Last four values are ignored in fixed mode.
 
-  char* buffer = reinterpret_cast<char*>(
-      Isolate::Current()->current_zone()->Allocate(kBufferSize));
+  char* buffer = Isolate::Current()->current_zone()->Alloc<char>(kBufferSize);
   buffer[kBufferSize - 1] = '\0';
   double_conversion::StringBuilder builder(buffer, kBufferSize);
   bool status = converter.ToFixed(d, fraction_digits, &builder);
@@ -121,8 +120,7 @@
       kDoubleToStringCommonExponentChar,
       0, 0, 0, 0);  // Last four values are ignored in exponential mode.
 
-  char* buffer = reinterpret_cast<char*>(
-      Isolate::Current()->current_zone()->Allocate(kBufferSize));
+  char* buffer = Isolate::Current()->current_zone()->Alloc<char>(kBufferSize);
   buffer[kBufferSize - 1] = '\0';
   double_conversion::StringBuilder builder(buffer, kBufferSize);
   bool status = converter.ToExponential(d, fraction_digits, &builder);
@@ -162,8 +160,7 @@
       kMaxLeadingPaddingZeroes,
       kMaxTrailingPaddingZeroes);
 
-  char* buffer = reinterpret_cast<char*>(
-      Isolate::Current()->current_zone()->Allocate(kBufferSize));
+  char* buffer = Isolate::Current()->current_zone()->Alloc<char>(kBufferSize);
   buffer[kBufferSize - 1] = '\0';
   double_conversion::StringBuilder builder(buffer, kBufferSize);
   bool status = converter.ToPrecision(d, precision, &builder);