Optimize functions containing try-catch.

runtime/vm/parser.cc

Index: runtime/vm/parser.cc
===================================================================
--- runtime/vm/parser.cc	(revision 22436)
+++ runtime/vm/parser.cc	(working copy)
@@ -6021,6 +6021,7 @@
     context_var = new LocalVariable(TokenPos(),
                                     Symbols::SavedTryContextVar(),
                                     Type::ZoneHandle(Type::DynamicType()));
+    context_var->mark_as_always_live();
     current_block_->scope->AddVariable(context_var);
   }
   LocalVariable* catch_excp_var =
@@ -6029,6 +6030,7 @@
     catch_excp_var = new LocalVariable(TokenPos(),
                                        Symbols::ExceptionVar(),
                                        Type::ZoneHandle(Type::DynamicType()));
+    catch_excp_var->mark_as_always_live();
     current_block_->scope->AddVariable(catch_excp_var);
   }
   LocalVariable* catch_trace_var =
@@ -6037,6 +6039,7 @@
     catch_trace_var = new LocalVariable(TokenPos(),
                                         Symbols::StacktraceVar(),
                                         Type::ZoneHandle(Type::DynamicType()));
+    catch_trace_var->mark_as_always_live();
     current_block_->scope->AddVariable(catch_trace_var);
   }