Redo "Constant fold charCodeAt via constant system"

sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart

 // 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.
 
 part of ssa;
 
 /**
  * [InvokeDynamicSpecializer] and its subclasses are helpers to
  * optimize intercepted dynamic calls. It knows what input types
  * would be beneficial for performance, and how to change a invoke
@@ skipping 51 matching lines @@
         return const EqualsSpecializer();
       } else if (selector.name == '<') {
         return const LessSpecializer();
       } else if (selector.name == '<=') {
         return const LessEqualSpecializer();
       } else if (selector.name == '>') {
         return const GreaterSpecializer();
       } else if (selector.name == '>=') {
         return const GreaterEqualSpecializer();
       }
+    } else if (selector.kind == SelectorKind.CALL) {
+      if (selector.argumentCount == 1 && selector.namedArguments.length == 0) {
+        if (selector.name == 'codeUnitAt') {
+          return const CodeUnitAtSpecializer();
+        }
+      }
     }
     return const InvokeDynamicSpecializer();
   }
 }
 
 class IndexAssignSpecializer extends InvokeDynamicSpecializer {
   const IndexAssignSpecializer();
 
   HInstruction tryConvertToBuiltin(HInvokeDynamic instruction,
                                    Compiler compiler) {
@@ skipping 637 matching lines @@
   }
 
   HInstruction newBuiltinVariant(HInvokeDynamic instruction,
                                  Compiler compiler) {
     JavaScriptBackend backend = compiler.backend;
     return new HLessEqual(
         instruction.inputs[1], instruction.inputs[2],
         instruction.selector, backend.boolType);
   }
 }
+
+class CodeUnitAtSpecializer extends InvokeDynamicSpecializer {
+  const CodeUnitAtSpecializer();
+
+  BinaryOperation operation(ConstantSystem constantSystem) {
+    return constantSystem.codeUnitAt;
+  }
+
+  HInstruction tryConvertToBuiltin(HInvokeDynamic instruction,
+                                   Compiler compiler) {
+    // TODO(sra): Implement a builtin HCodeUnitAt instruction and the same index
+    // bounds checking optimizations as for HIndex.
+    return null;
+  }
+}