Revert "Constant fold charCodeAt via constant system"

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

Index: sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
diff --git a/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart b/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
index 5511118ee603a6cf9148ffdf3f5150b14cfe79f6..d9fc2865689090dbc84f51569946e071dfa5f528 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
@@ -69,12 +69,6 @@ class InvokeDynamicSpecializer {
       } 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();
   }
@@ -732,18 +726,3 @@ class LessEqualSpecializer extends RelationalSpecializer {
         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;
-  }
-}