Fix Windows build by removing an overly strict assertion.

runtime/vm/object.h

Index: runtime/vm/object.h
===================================================================
--- runtime/vm/object.h	(revision 36989)
+++ runtime/vm/object.h	(working copy)
@@ -3628,7 +3628,6 @@
   static const intptr_t kMaxElements = kSmiMax / kBytesPerElement;
 
   static intptr_t InstanceSize() {
-    ASSERT(sizeof(RawCode) == OFFSET_OF(RawCode, data_));

[Ivan Posva, 2014/06/04 15:53:14]

There are too many things riding on the assertion that sizeof(RawCode) gives us
the correct information. We will have to solve this differently.

[Florian Schneider, 2014/06/06 09:13:03]

Yes, this is fragile: InstanceSize(intptr_t len) needs to be correct. This means
that RoundedAllocationSize must match what the C++ compiler produces. There are
differences betwwen MSVC and g++: sizeof(RawCode) is 72 on my Linux machine, and
80 on Win32.

OFFSET_OF(data_) is 72 on Linux, and 76 on Win32.

In this instance the RoundedAllocationSize would be still 80 since we round up
to multiples of 8 (kObjectAlignment), but there is no guarantee that these
things match.

     return 0;
   }
   static intptr_t InstanceSize(intptr_t len) {