Add contexts.

runtime/vm/flow_graph_builder.h

Index: runtime/vm/flow_graph_builder.h
===================================================================
--- runtime/vm/flow_graph_builder.h	(revision 5792)
+++ runtime/vm/flow_graph_builder.h	(working copy)
@@ -21,7 +21,8 @@
   explicit FlowGraphBuilder(const ParsedFunction& parsed_function)
       : parsed_function_(parsed_function),
         preorder_block_entries_(),
-        postorder_block_entries_() { }
+        postorder_block_entries_(),
+        context_level_(0) { }
 
   void BuildGraph();
 
@@ -145,6 +146,11 @@
   Value* BuildInstantiatorTypeArguments(intptr_t token_index,
                                         intptr_t start_index);
 
+  bool MustSaveRestoreContext(SequenceNode* node) const;
+
+  // Moves parent context into the context register.
+  void UnchainContext();
+
   void CloseFragment() { exit_ = NULL; }
   intptr_t AllocateTempIndex() { return temp_index_++; }