Redo "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 d9fc2865689090dbc84f51569946e071dfa5f528..5511118ee603a6cf9148ffdf3f5150b14cfe79f6 100644
--- a/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
+++ b/sdk/lib/_internal/compiler/implementation/ssa/invoke_dynamic_specializers.dart
@@ -69,6 +69,12 @@ 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();
   }
@@ -726,3 +732,18 @@ 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;
+  }
+}