Move CHA field back to Isolate, pending a permanent fix.

runtime/vm/thread.cc

Index: runtime/vm/thread.cc
===================================================================
--- runtime/vm/thread.cc	(revision 45029)
+++ runtime/vm/thread.cc	(working copy)
@@ -87,4 +87,15 @@
 }
 
 
+CHA* Thread::cha() const {
+  ASSERT(isolate_ != NULL);
+  return isolate_->cha_;
+}
+
+
+void Thread::set_cha(CHA* value) {
+  ASSERT(isolate_ != NULL);
+  isolate_->cha_ = value;
+}
+
 }  // namespace dart