Avoid integer overflow when computing the size of new space in bytes.

runtime/vm/heap.cc

Index: runtime/vm/heap.cc
diff --git a/runtime/vm/heap.cc b/runtime/vm/heap.cc
index 70e6a5e42eb9e566ab079761944f839b2fe25013..1e81aaff491b5b78462ff62335dfbbb28aee319a 100644
--- a/runtime/vm/heap.cc
+++ b/runtime/vm/heap.cc
@@ -36,7 +36,7 @@ DEFINE_FLAG(int, code_heap_size, Heap::kCodeHeapSizeInMB,
             "code heap size in MB,"
             "e.g: --code_heap_size=8 allocates a 8MB code heap");
 
-  Heap::Heap() : read_only_(false) {
+Heap::Heap() : read_only_(false) {
   new_space_ = new Scavenger(this,
                              (FLAG_new_gen_heap_size * MB),
                              kNewObjectAlignmentOffset);