VM: Don't rely on always running constant propagation in the optimizer

runtime/vm/flow_graph_builder.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/flow_graph_builder.h"
 
 #include "lib/invocation_mirror.h"
 #include "vm/ast_printer.h"
 #include "vm/bit_vector.h"
 #include "vm/compiler.h"
@@ skipping 530 matching lines @@
                                       new(Z) Value(true_const),
                                       new(Z) Value(true_const),
                                       false));  // No number check.
     branch->InheritDeoptTarget(zone(), call_);
     *branch->true_successor_address() = callee_entry;
     *branch->false_successor_address() = false_block;
 
     call_->previous()->AppendInstruction(branch);
     call_block->set_last_instruction(branch);
 
+    // Replace uses of the return value with null to maintain valid
+    // SSA form - even though the rest of the caller is unreachable.
+    call_->ReplaceUsesWith(caller_graph_->constant_null());
+
     // Update dominator tree.
     call_block->AddDominatedBlock(callee_entry);
     call_block->AddDominatedBlock(false_block);
 
   } else {
     Definition* callee_result = JoinReturns(&callee_exit,
                                             &callee_last_instruction,
                                             call_block->try_index());
     if (callee_result != NULL) {
       call_->ReplaceUsesWith(callee_result);
@@ skipping 4108 matching lines @@
   Report::MessageF(Report::kBailout,
                    Script::Handle(function.script()),
                    function.token_pos(),
                    "FlowGraphBuilder Bailout: %s %s",
                    String::Handle(function.name()).ToCString(),
                    reason);
   UNREACHABLE();
 }
 
 }  // namespace dart