Avoid array bounds check when allowed by guarded field.

runtime/vm/intermediate_language.h

 // Copyright (c) 2013, 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.
 
 #ifndef VM_INTERMEDIATE_LANGUAGE_H_
 #define VM_INTERMEDIATE_LANGUAGE_H_
 
 #include "vm/allocation.h"
 #include "vm/ast.h"
 #include "vm/growable_array.h"
@@ skipping 6470 matching lines @@
 
   virtual bool MayThrow() const { return false; }
 
  private:
   DISALLOW_COPY_AND_ASSIGN(CheckSmiInstr);
 };
 
 
 class CheckArrayBoundInstr : public TemplateInstruction<2> {
  public:
-  CheckArrayBoundInstr(Value* length, Value* index, intptr_t deopt_id) {
+  CheckArrayBoundInstr(const Field* array, Value* length, Value* index,
+                       intptr_t deopt_id) : array_(array) {
     SetInputAt(kLengthPos, length);
     SetInputAt(kIndexPos, index);
     // Override generated deopt-id.
     deopt_id_ = deopt_id;
   }
 
   Value* length() const { return inputs_[kLengthPos]; }
   Value* index() const { return inputs_[kIndexPos]; }
 
   DECLARE_INSTRUCTION(CheckArrayBound)
@@ skipping 18 matching lines @@
 
   virtual bool MayThrow() const { return false; }
 
  private:
   // Give a name to the location/input indices.
   enum {
     kLengthPos = 0,
     kIndexPos = 1
   };
 
+  const Field* array_;
+
   DISALLOW_COPY_AND_ASSIGN(CheckArrayBoundInstr);
 };
 
 
 #undef DECLARE_INSTRUCTION
 
 class Environment : public ZoneAllocated {
  public:
   // Iterate the non-NULL values in the innermost level of an environment.
   class ShallowIterator : public ValueObject {
@@ skipping 208 matching lines @@
   ForwardInstructionIterator* current_iterator_;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(FlowGraphVisitor);
 };
 
 
 }  // namespace dart
 
 #endif  // VM_INTERMEDIATE_LANGUAGE_H_