Double-align Zone allocations to fix MIPS issue.

runtime/vm/zone.h

Index: runtime/vm/zone.h
===================================================================
--- runtime/vm/zone.h	(revision 42786)
+++ runtime/vm/zone.h	(working copy)
@@ -67,6 +67,7 @@
       large_segments_(NULL),
       handles_(),
       previous_(NULL) {
+    ASSERT(Utils::IsAligned(position_, kAlignment));
 #ifdef DEBUG
     // Zap the entire initial buffer.
     memset(initial_buffer_.pointer(), kZapUninitializedByte,
@@ -84,7 +85,7 @@
   }
 
   // All pointers returned from AllocateUnsafe() and New() have this alignment.
-  static const intptr_t kAlignment = kWordSize;
+  static const intptr_t kAlignment = kDoubleSize;
 
   // Default initial chunk size.
   static const intptr_t kInitialChunkSize = 1 * KB;
@@ -121,7 +122,8 @@
   // This would act as the initial stack allocated chunk so that we don't
   // end up calling malloc/free on zone scopes that allocate less than
   // kChunkSize
-  uint8_t buffer_[kInitialChunkSize];
+  COMPILE_ASSERT(kAlignment <= 8);
+  ALIGN8 uint8_t buffer_[kInitialChunkSize];
   MemoryRegion initial_buffer_;
 
   // The free region in the current (head) segment or the initial buffer is