Revert r28050 due to crashes with --optimization-counter-threshold=5.

runtime/vm/flow_graph_compiler.cc

Index: runtime/vm/flow_graph_compiler.cc
===================================================================
--- runtime/vm/flow_graph_compiler.cc	(revision 28051)
+++ runtime/vm/flow_graph_compiler.cc	(working copy)
@@ -170,6 +170,13 @@
 }
 
 
+static bool IsEmptyBlock(BlockEntryInstr* block) {
+  return !block->HasParallelMove() &&
+         block->next()->IsGoto() &&
+         !block->next()->AsGoto()->HasParallelMove();
+}
+
+
 void FlowGraphCompiler::CompactBlock(BlockEntryInstr* block) {
   BlockInfo* block_info = block_info_[block->postorder_number()];
 
@@ -179,7 +186,7 @@
   }
   block_info->mark();
 
-  if (block->IsEmptyBlock()) {
+  if (IsEmptyBlock(block)) {
     // For empty blocks, record a corresponding nonempty target as their
     // jump label.
     BlockEntryInstr* target = block->next()->AsGoto()->successor();